Is the ticket issued straight away after I pay for a flight booking, or does it take time?

In most cases yes, the ticket is issued within a minute or two of the payment going through, and the airline reference comes back into your booking screen almost immediately. Flight booking APIs are built so that the search, the booking and the ticketing all happen in one flow, so for the large majority of bookings you can send your customer their ticket while they are still on the phone with you.

There are situations where it takes longer. Some airlines and some fare types put the booking into a pending state while their own system checks it, and that can take a few minutes or, more rarely, a few hours. Group bookings, fares that need a document to be verified, and some specially negotiated fares are the usual reasons for a wait. In those cases the booking sits in your system as pending rather than confirmed, so you and your staff can always see exactly where it stands instead of guessing.

It is only fair to be clear that the timing is in the airline's hands, not ours. If an airline is slow to return a ticket number, no software can speed that up. It also depends on how your own setup is arranged, because some agencies choose to have bookings issued automatically and others prefer their office to check the payment first before the ticket goes out, and that extra step naturally adds a little time.

Before you go live, ask about which airlines in your market ticket instantly and which tend to go pending, and make sure your team knows to watch a pending booking rather than make a second booking for the same traveller.