In most cases it is confirmed straight away, but not every room works that way. The majority of rates that come back in a search are live inventory, which means the supplier already holds the rooms and can confirm the moment you complete the booking. For those you get a confirmation reference back almost immediately and you can send your customer the voucher the same day.
Some rates are what the industry calls on request. With these the supplier has to go back to the hotel and check before anything is guaranteed, so the booking sits in a pending state for a while. That can be resolved in a few hours, or it can take until the next working day, and in a small number of cases the hotel says no and the request is declined. Our hotel booking API returns the status with the booking, so you can see whether you are holding a confirmed reservation or a request that is still waiting.
Which rates are instant and which are on request is decided by the supplier and the hotel, not by us, and the mix you see will depend on the suppliers connected to your plan and on the property itself. Smaller independent hotels and some negotiated rates are more likely to be on request than large chains. Payment handling for a declined request also follows the supplier's rules and the way your payment setup is arranged, so it is worth knowing that in advance.
Before you go live, check the booking status that comes back in the reply and make it a rule in your team never to send a voucher until it reads confirmed, and ask our support team which of your connected suppliers carry on request rates so you can set the right expectation with your customers.