TL;DRAll turret types should have the ability to sleep when no enemies are near.
What ?Looking at the entity time usage debug stats, I can see that artillery turrets and flamethrower turrets are using a small but meaningful amount of time. This is on a map which currently has no biters at all (I occasionally turn them off to focus on building, then turn them back on to make sure the defenses are airtight).
Laser and bullet turrets are both capable of sleeping and not using any UPS when there are no enemies near. It would make sense for flamethrower turrets to be able to do the same thing. They have a maximum possible radius of effect, just like the other turret types, and when nothing is near they can just wait to be woken up.
Presumably artillery turrets can stay asleep unless an enemy builds a targetable object within its range. I've seen enemy artillery turrets target players, though, so perhaps this is a bit more complicated.
Attached is an image showing 44 artillery turrets taking 0.025s and 345 flamethrower turrets taking 0.035s of the update, despite there being nothing for either type of turret to target.