Honestly, it's a sad thing, really. You should not have to put a timer on a request for service. The length of time between the request and the requested date is unimportant. There is no excuse for that. An initial email should be answered within 12 hours (and that's too long!), allowing for unavailability at any time of day or night. A lady would be stupid to assume that the appointment is valid with no communication prior to the date. If I were a guy, I would DNS any lady who has poor communication skills.
Personally, I wouldn't suggest a statement like that as a measure, as it indicates you're not used to getting responses to your requests, but I would definitely have a Plan B if Plan A is a retard and doesn't check/respond to her emails.
Good to hear from you, CT!