so far i have had a purple tang for the last 5 yrs, blue damsel for 6 yrs, and a tomato clown for 5 yrs, now the average will change from tank to tank but some fish will live for more than 10 years if the tank is properly cared for.
Small reef fishes have a relatively short lifespan...like 5 years........still a long time......I think angels and butterflies always are the ones that break the longevity record. they can easily live more than 10 years if WELL cared for....also true for triggers and tangs..... I heard about large angels having lived more than 20 years in public aquariums.
Do a search. There are a few threads on this board mentioning life span of fishes. From what I've read clowns can live close to 20 years and tangs can live up to 40 years. I've not experienced this myself as my tank is not quite a year old yet, but I look forward to having my 2 percs for the next 15 - 20 years.