My players encountered some wooden golems, which are vulnerable to fire damage. One of the players casts a Fire Bolt spell, the description of which says "A flammable object hit by this spell ignites if it isn't being worn or carried." So the next question is, after a wooden golem is hit twice with a fire bolt, how long should the thing continue to take damage due to being on fire?

I know the default answer is — whatever the DM wants it to be, but I'm taking the temperature of this community (pun intended). My first impulse is to say 1d6 rounds. Does anyone have a more compelling idea?