It depends on lots of things--the size of the fish, how hot the fire was, how close to the fire.
We used to cook fish by nailing them to a board. Gut the fish first, then nail both head and tail to a board and angle it, fish-side-down, over the fire. But you can also put it on a grill over the fire, and in a way that's better because you can turn it over then. It takes about 10-20 min. You can poke the fish with your finger and tell if it's done. If it's underdone, that's okay, it doesn't really take much to cook a fish, and under-cooked is better than over-cooked.