12 years
As a guest, I think there are quite a few reasons why I might choose to book for (say) 2 nights, then, after arrival, ask to book one or two more.
I would say: keep it simple to avoid confusion. Namely, charge them at the time when they actually reserve a bed, whether for 1 night or several. But also, be generous and be ready to refund if they book (say) 3 nights then cancel the 3rd one.
Log in to join discussion