Can the hotel booking API confirm a room straight away, or do some hotels take time to come back to me?

Yes, most bookings made through a hotel booking API are confirmed straight away, but not every one of them is, and it helps to know which is which before you promise a room to a customer.

Most rates come back as instant confirmation. You send the booking, the supplier checks with the hotel while you wait, and a confirmation number comes back within seconds. From that moment the room is held and your customer can be told the booking is done. A smaller number of rates are what suppliers call on request. There the hotel has to agree first, so the booking is accepted as pending and the real answer arrives later, sometimes within a few hours and sometimes the next working day.

The API usually tells you which kind you are dealing with, either in the search results or in the response you get when you book, so your staff can see it before they speak to the customer. How clearly it is marked depends on the supplier, and some only make it obvious at the booking step. Now and then a rate that looked instant still fails, because the last room went while your request was on its way, so your process should treat the confirmation number, and not the fact that you sent a booking, as proof that the room is held.

Ask each of your suppliers whether their rates include on request hotels, and set your portal up so the confirmation email only reaches the customer once that confirmation number has come back.