Your probably going to get alot of different answers on this on but, I usually keep using the same oil for frying fish until it just starts smelling too fishy. Once it reaches that too fishy point, I'll throw it out but I always use alittle of the old oil in with the new oil (in the skillet, not in the container) that helps it brown the fish better. I've kept using the same oil for about 3 months but there again, its depends on how much fish ya fry. Confusing I know but, thats my 2 cents.
Bluegill