Airports of Thailand provides a free shuttle between Suvarnabhumi and Don Mueang for eligible connecting passengers who can show onward travel documents.
Who can use the free shuttle
The airport states that it is for passengers transferring between BKK and DMK, not general public transport.
Operating window
The official BKK transport page lists the airport shuttle from 05:00 to 24:00, with scheduled frequency information.
Build a large buffer
This is a cross-airport transfer across Bangkok road traffic. Include immigration, baggage, shuttle wait, road travel, check-in and security at the second airport.
This is an airport change, not a terminal transfer
BKK and Don Mueang are separate Bangkok airports. The free shuttle is intended for eligible transferring passengers who can show onward travel, but the road journey is still exposed to traffic. Build in arrival formalities, baggage if required, the cross-city transfer, the second airport’s check-in cutoff and security; the bus timetable alone is not a safe connection estimate.
Sources and verification
Time-sensitive airport, airline, visa, security and transport details should always be confirmed with the relevant official source before travel.