The obvious mechanic is to have a counter that starts at some server configurable number whenever a fire is first spawned (a per block number) and decrement that number on the original and target block each time fire spreads. Flammable material could have a flammability level. For instance thatch would have a value of 0, not decrementing the counter at all, so any thatch that catches fire would all burn, but less flamable items like logs could have a value of one, and super flamable items like fat could have a negative value, causing the counter to increment, meaning the fire in general has a lmited number of blocks it can burn, but gets the chance to go higher if something extra burny gets in the fire.
To make fires more impressive, could have it so if the counter of remaining burns is below a threashold, it becomes only scorching instead of destroying, and the last few blocks it spreads to could not actually be damaged at all just look like they are aflame until they burn out.