I'll email back asking for them to call. Until then it's open unless we've agreed a future time/date to speak. Sounds harsh but some guys don't get back to you.
I'm the same here. I'm happy to take email bookings til I'm blue in the face but if they haven't called to confirm by midday on the day then I ain't putting my stockings on! And if somebody else calls after midday, they can get the appointment.
I actually don't blacklist clients that email for an advance booking but don't call to confirm, although I do blacklist a phone-booking or confirming client who doesn't show up (obviously). I say in response to their email that I've got their appointment in my diary but that they need to call to confirm in order to get my location etc; I think some guys honestly do make appointments that they want but on the day can't get to due to work/family/etc and since if I don't get the confirmation I don't get ready or waste any of my time, it's not a problem to give them another chance.
I don't work every day though and have the luxury of a super-flexible schedule, so I think I'd be less lenient about confirming advanced bookings if I could was only available on, say, one day a week. If that was the case I'd say that I'd pencil them in but they'd need to confirm before 9am on the day to keep their appointment!