If my customer cancels a flight booked on my portal, will they get their money back quickly?

No, a refund is almost never instant, and the honest answer is that it usually takes weeks rather than days. It is worth telling your customer this at the moment they cancel, because most people expect the money back the same week.

When your staff cancel a ticket on the portal, the request goes to the airline through the API and you get a confirmation that the cancellation has been accepted. That part is quick. What follows is the airline working out how much of the fare is actually refundable once its own penalty and the taxes it is willing to return are taken into account. That figure comes back to you, and only then does the money begin to move. Airlines vary a great deal here, and some are far slower on certain routes and fare types than others.

The money also does not travel straight from the airline to your customer. It goes back to whoever paid the airline, which is normally your supplier or your own account, and you then refund your customer by the method they originally used to pay you. If they paid by card, their own bank adds a few more days at the end. On a non refundable fare only the taxes come back, and on some fares not even those.

The safest approach is to cancel on the portal first, note the refundable amount the airline actually returns, then give your customer that figure and a realistic window rather than a fixed date, and keep the case open on your side until the money has landed with them.