If an airline changes the time of a flight I have already booked, will I find out through your flight booking API?

Yes, in most cases you will be told, but how fast the news reaches you and how much detail comes with it depends on the airline and on the supplier the ticket was issued through.

When an airline moves a departure time or cancels a flight, it usually sends a schedule change message against the booking record. That message flows back to your booking, so when you open it you see the new times, or you see the flight marked as changed or cancelled rather than the original details you sold. Depending on your plan, changed bookings can also be flagged in your booking list so you are not opening each one to hunt for problems, and in some setups an alert can be sent to your own team inbox.

It is fair to say this is not perfectly even across airlines. Full service carriers are generally good about sending structured change messages. Budget carriers often notify the passenger contact on the booking directly instead, so what you receive may be thinner. Same day disruption, such as a delay or a diversion on the morning of travel, is handled by the airline at the airport and will not always reach your system in useful time, so your customer may hear it before you do.

The practical thing to do is to ask us which change alerts are available on your plan, make sure the contact details entered at the time of booking are ones you actually monitor, and build a habit of reviewing upcoming departures shortly before travel so nothing sits unnoticed.