Newbie - FAQ
Some people are more (or less!) patient than that, but I'd allow a minimum of 24 hours. Sending a follow-up message, just in case she never received the first one, is often recommended. 
As with any aspect of this activity, it always helps if you've really done your homework. Did her reviews mention anything about pre-date communication issues or did everyone say she got back to them right away? If she has a schedule on her site, does it show she's available now? If she is listed as being off, she might not check her messages again until she's back on the job. Is she on P411? If so, look to see when she last logged in. If it's been a week or more, then she might be MIA for the moment. Hope that helps, and I hope she gets back to you!
Some people are more (or less!) patient than that, but I'd allow a minimum of 24 hours. Sending a follow-up message, just in case she never received the first one, is often recommended.
Only you can decide what is an acceptable wait time for you, but I suggest giving the lady a timeline (ie: deadline) such as: I have a very limited window, so I need to confirm this no later than such-and-such time or I won't be able to make it. Assuming she's checking her email, and reads yours, that's a great way to inspire a prompt response.
If she has a schedule on her site, does it show she's available now? If she is listed as being off, she might not check her messages again until she's back on the job.
Hope that helps, and I hope she gets back to you!