Follow what you think reasonable (given your time circumstance)-and stick with the one you book with and don't look back !  If/when the other one or ones you called call you back-tell them the truth!
Perhaps you should include in your voice message or email/text some sort of time frame and an upfront apology if they get your message after your time limit or whatever!

There's no set time for me between contacting multiple providers. I have windows of opportunity that vary. I recall the first time this situation happened with me:

When TER and I were younger, I had a sudden windfall in time and a scratch off ticket, so I said "Oh Boy! I can see a provider. Who to choose? Lady #1!" I quickly emailed her and didn't hear back. Next morning, knowing my window of opportunity was closing, I emailed Lady #2 who gave me got a quick response (awesome!)... only to hear back from Lady #1 later that same day giving a good reason why she had been unable to reply the previous day.  

The way I handled things was to reach out with a quick DM to a more experienced, trusted reviewer and made my case. He agreed that since I (still) have a semblance of a conscience, then my deal with Lady #2 should stick. I emailed Lady #1, said "sorry dear, but I made other plans and I try to keep a date once it's made". Lady #1 was professional in her response in understanding - which made my desire to see her even more so. Lady #2 was wonderful. I eventually saw Lady #1 and we shared a delightful time as she admitted that men are as flaky as women when it comes to the better deal, but respected me a bit more - which actually meant something to me.

So "let your conscience be your guide". If there's any chance at all that you'd feel guilty about cancelling #2 to see #1, I'd say keep the date with #2. Otherwise you risk distracting thoughts while you are enjoying #1. Likewise, if you are not honest with yourself, you might see #2 and be distracted by thoughts of #1.  In either case, you might learn something new about yourself.

My humpday wisdom,
ga_kosh

I usually wait no more than a few hours before go to an alternative. I requested an appointment with a provider in the morning and after 4 hours I did not hear from her so I made an appointment with someone else. I heard from the original provider after 5 hours and I told her I made other plans and explained my situation.  She told me that my timeframe was unfair and unreasonable and would not agree to see me at a later date. I have found that agencies or providers that have schedulers are more responsive than independents.  just my experience.

Book them both as a double.

Problem solved, just book them both if they are into doubles.

I never wait.

The first one to respond to me wins my business and money.

I think some of you have forgotten that this is a business here. Nothing more, nothing less.