You need to accept or decline within 24 hours. If you decline she can request again. Or she can withdraw her request now and request again when she is ready. If you want to trust her you can block those dates and tell her to message you to unblock them when she is ready to book. Lots of times these type of requests never end up booking and paying so you should ask yourself if you want to take the chance of missing a certain booking for a possible booking. It may be best to tell her you can't hold the dates for her without payment, but you would be happy to approve her when she is ready, if the dates are still open.
If her request was a request instead of an inquiry and you accept it may book her and then the system may cancel her if the card is not valid. So she should withdraw her request if she wants to wait. It is not really your concern if she is not ready to book but went ahead and made the request anyway.
If it was an inquiry and you pre-approve then she would have 24 hours to book or the whole thing would expire and she would have to start over when she is ready to pay.
Kerrin