The guest has 24 hours to book after your pre-approval. After the 24-hour period, if the guest tries to book, you have another opportunity to accept or decline. At least, that's my understanding. I find some of the behavior of pre-approvals quite frustrating, and there are definitely some bugs related to them.
I also find them an annoying waste of time, as the pre-approval requests seem to come mostly from window-shoppers who rarely get back to me. I think they send pre-approval requests to multiple hosts, see who accepts them, and then pick one later.
Also, in case you're worried, a pre-approval (unless booked) doesn't appear in your calendar or block the dates.