3 weeks is way too long, but a day isn't long.
I've had guys schedule that day and if you don't write back as soon as they feel is right. Then they will schedule elsewhere. Had that happen before, someone made an appointment & was told to get back with that person at a certain time within 24 hours and they made plans anyways.
Give her 24 hours, not 3 weeks But if you book another date within the time frame you told her to get back with you, then it's on you and she has every right to be upset and can chose not to deal with that person again.