@Daniel ***********
Quickest I've done it is 30 minutes a couple months ago from landing to the luggage belt. Longest it's ever taken me is 2h many years back. Then from the intl' floor to the domestic departures one you have like 10 minutes, plus queue etc. Allocating 4h is not wild. You might end up with a full 2h to kill but hey better that than missing a flight. Chiang Mai ain't going anywhere anyway hah.
I wouldn't go the buy same day route bc depending on it you'd be surprised, most flights could be full