Not a stupid question at all. There are several enviromental and health issues that determine a fish's life span. There is no simple answer to your question. Life for a fish is determined by its lifestyle. The warmer the water the quicker the metabolism works and ergo the shorter the lifespan. Different fish expire at different rates. Smaller the fish the quicker the expiry.
So if we are talking a utopian life for small fish such as tetras, rasporas, you can expect 3 - 4 years. Angels 5-8 years, some have lived 10 years.
I have a friend who has a marbled angel in water as green as pea soup, he doesn't do many water changes and just has the one fish. The angel is 10 years old. I don't advocate that anyone treats their fish in the same manner as my friend, but i used him as an example.
