Burn time of wood is quite random. Depends only on wood object’s volume and fire’s size.
There is a minor exploit: you can start a fire with a stick, then craft said burning stick into a cudgel. The cudgel will not be burned and will fuel the fire for longer.
Fires with nothing to burn will quickly die out, especially if they’re big.
A fire that burns a single small (<=10 volume) piece of wood should be stable (except in the rain) as the stick adds 1 “fuel” per turn and the fire burns 1 “fuel” per turn.
If you add a second stick, it will gain 1 fuel per turn. Once it has nothing to burn, it will lose 3 fuel per turn (if small, 5 and 7 if medium or raging).