I'll be in Vegas in a couple weeks and will have most of my days, and some nights free. I'd like to set up time with a provider or two but I've never done this before. Should I wait until a couple days beforehand or is it best to set it up ASAP? Do providers book out that far?

For me, The more advanced the better that way I can plan my schedule and it also give you the best chance to get the date and time you wish to meet.  

First of all, a big welcome to the realm of eroticism!  

Book at least 2 hours, preferably 3 or you run the risk that your provider will get a better offer and cancel on you.

Booked a 4 hour dinner date recently and got no showed/no texted on. Probably bait and switch as opposed to a better offer in my particular case, but length of appointment is no guarantee that she'll show.

If you know your schedule it is best to reach out to lady or ladies you are interested.
As some of us travel and might not be available during your visit or time of availability.
Also many of us have alternate ways of screening for gents who do not have references.
I welcome advanced booking I have gents who have already booked for Nov and Dec.
Enjoy your trip here in Vegas.

If there is a lady who catches you eye book out. 6 hr dinner date vs an hr. Who might get preference.

In my experience book a week in advance.. get the screening out of the way.. Confirm a couple of days prior and the morning off. Booking more than a week in advance and you have a greater chance of the lady canceling. DO CONFIRM a couple of days prior and on the day of though. My past few trips have not been good with a lot of ladies canceling and at least if this happens you can try and book with someone else. Just my two cents from my experiences lately. Have not returned to Vegas in a few months because of this.

I agree with most of what Jaysurfer said. I was in LV two weeks ago and had booked a two-hour date with a highly rated and very reputable LV lady two weeks in advance. As I often do, I confirmed the date in the morning. Lo and behold, the lady's assistant sent me her cancellation Email 30 minutes later stating she would not be available for a week. The next day, that same lady posted pictures on her Twitter account about the great time she was having with another gent, the better deal. The objectionable move here was that the booker had not made the tiny effort of letting me know on their own initiative.  
NM, I had a super time regardless that same night with Layla Banks (highly recommended) and later that week with both Layla and her friend Emi Rei (highly recommended). Guys, there are a lot of great ladies to meet, especially in LV with over 1,000 providers actively advertising their services. Don't tell me that out of that list there is only one for you, you are not getting married to her for Pete's sake. Don't let your hormones take control, get yourself locked into a few polished/staged photos on a website, and end up with blue balls or a temper tantrum. Be a man and move on to another lady. A real man knows he has plenty of options in LV and welcomes the adventure.