I would prefer to deal directly with the provider and wait a little but not everyone's life is the same so taking that into account if I have to deal with an assistant I'll do it.  I like the personal touch and as you said a bad assistant can really screw things up.  That being said I think every inquiry should be responded to in 24 hours if not the same day if one reach out in the morning.  I respond to all inquiries to my company within 24 hours and so should a successful provider who is running her business.  Good customer service starts with the initial response.  Granted what happens during the session is the most important portion of our involvement together but doing everything professionally and in a timely fashion counts for something as well.  I plan my sessions well in advance so I don't need a response in a couple hours but I get the sense that many are more spur of the moment than I and may need that.  When I reach out to a provider my intention is to make an appointment and provide funds to her business so I expect to be treated in a manner that shows some appreciation.  Unfortunately the "hi what's up" and the serial last minute cancelers of the world are getting in my way by filling their inboxes with crap.

Everyone likes a quick response either by the girl herself or an assistant even though most prefer the girl because it seems more personal, and some think there talking to the girl when its the assistant. I actually like to send a follow up email to say i enjoyed the time and does the girl with the assistant read it or gets it forward to her. Also think the girls should be more proactive if the client was good and say come back soon or whatever in an email if not in person. We appreciate that more than you know

By "timely" I mean within 24 hours, give or take a couple. (assuming that I am scheduling at least few days out) If the reply is from an assistant, that's just the way it goes. (The auto-reply doesn't count as a response IMO)
I don't send very many "hi, what's up?" emails, but if I do and I don't receive a reply, I don't take it personally. Mainly because I didn't get into this hobby because I wanted a pen-pal. And obviously you ladies are not getting paid to be  pen-pals

I started on my own but I was overwhelmed with all the extra stuff associated with this, not to mention I was still trying to figure out how to screen.  

I was dropping the ball by forgetting emails I needed to respond to, answering questions later when I got the info, remembering who was who, etc... so it just makes sense for me to have an assistant.

I instruct her to disclose she is not me when emailing clients, but sometimes she forgets. I try not to talk directly with clients because a client told me it was confusing when emailing with both of us, even though we were clear who we were each time, but I can understand how it can get tough. Especially when in the same email string. I'm her backup though, so if I see something is missing, or someone emailed with something specific she doesn't know the answer to, I try to take care of those.

I don't know how the other girls do it. I'd LOVE to be able to handle it all on my own

I would have to say I would prefer to hear from the gal herself in 24hrs or so (always love it when it is a lot less).  If I didn't hear from her in 48hrs and she wasn't someone I had seen before, I would probably move on (although I would probably wait longer if it was someone I was really wanting to meet as I don't normally schedule anything even remotely short notice).  

I have communicated with 2 assistants in my experiences so far and they were both pleasant and made it very clear that they were the gals assistant, but it isn't quite the same.  I would expect any response from an assistant to be extremely fast.

Personally, I really can't imagine sending a general "hi, what's up?" e-mail to someone I haven't met.  I would not expect a response.

If it took you longer than you prefer in your response time to get to me, just say something to that effect and that you apologize for the late response.  I don't really need any in-depth info on why.  If I come to find out it is the norm for you from future appointment requests I make, I will decide if I wish to deal with that or not.

Some people have the admin gene, and some don't, and fluctuating volume compounds the issue.  I regret that we don't know each other yet Mindy, but we have exchanged emails, and you have a wonderful presence in your correspondence.  So my suggestion is, because you care, use an assistant -- and this must be the key -- who has a really good system for sorting out the urgent/important and flagging those emails to you.  Then maybe you add a comment or two that has nothing to do with admin, and everything to do with "I'm really looking forward to this.  I can't wait for 'Betty' to set it up."

Two other things:

At least half the time I am a spur of the moment guy.  If I get an auto reply there's less of a chance I will follow through.  So then I need a quick screen or a pre-screen, somehow.  

Recently I saw someone by way of a local assistant.  I was a bit pissed that the assistant was passing herself off as the provider.  I would probably have repeated by now apart from that.

On the other hand, there is an assistant in the cities who I would definitely email to ask for help because she's real and smart and nice.  In fact I think she would be fun to take out to dinner (you know who you are)

My preference is for direct contact with the lady.  I was pretty firmly against dealing with assistants until one of our local ladies sent me a PM explaining why she started to use one.  That shed a whole different light on the subject for me.  Now I'm not so against it, but still prefer to communicate directly with the lady.  It helps me get a feel for who she is if I haven't met her yet.

As to responses to requests it seems to be all over the board, but I do expect a response of some kind within 48 hours.  If not I assume she is too busy and getting an appointment will be difficult OR she's simply not interested. I don't send "hi, what's up" type of messages.  If I request an appointment thru P411 I find I get a pretty quick response most of the time.  If not, I simply move on.  If using email I make sure the subject line tells her why I sent it. One subject line that I find works pretty well is "Appointment Request".

So at the end of the day my main concern is a reasonably prompt response whether directly, through an assistant, or through an automated reply.  If subsequent communications then are not reasonably prompt I move on.  My time is as valuable as hers.  

As others have said, it's a business which should be treated as such in a courteous manner by all parties involved.

And if I've handled my end well......then I think the customer should come(Sp?) first.

If I could use one to schedule meetings for me I would.   Phone tag or waiting for response on P411 is not fun.  If I need to move on,  I want to know right away.    

If using one is the best way for use to keep up with the volume of emails you get then do what you think is best so long as we know you are using one.

Not to Brag.... But...

I totally get to now..  
When you have the right Personal Assistant especially one that has a background in marketing and business development..
When you find the one that connects with you and knows how to work with your clients ..
Someone who you can collaberate with as a partner in your business and can formulate a Marketing Strategy with..
Keeps Your business going so you can focus on what really matters AKA the time you spend with your clients  
Great AWESOMAZING FANTASTICAL AMAZABALLS Things can happen ..

But dont take my word for it,, Let me Quote the interview one of my clients just gave

"TER: Anything you would like to include?  
Melaniee: I'd like to give a special shout out and thank you to my assistant Hanna, she has done a fantastic job working for me, my clients love her and she was the one who encouraged me to go for Ms. TER and I would not have been able to do it without her. "

*holds up a glass of celebratory champagne*

Cheers !!  

~Cat
~Every Hero Needs a Sidekick

Assistants basically guarantee you a visit but they also tend to overbook and double book the girl. Also if you become a regular to the girl and want to start sending her more flirtatious emails the assistant will intercept them and the girl may never see them. The really good girls will give you another email for you to contact them.
  Now not having assistants has its pluses as you actually get to talk the girl and the emails can be fun and flirtatious.
  As for the bad as OP states they may not always get back to you and it does become frustrating if you really want to see them as I know first hand. I guess you just have to be patient and wait your turn but emailing the actual your just taking a chance and hoping for the best.