Hi
I find longevity in fish in general an interesting topic as I don't see how it would be possible to arrive at" normal" for a wild fish. longevity of any creature is related to metabolism which is related to temps particularly with cold blooded animals. note the remarkable life spans of turtles for example??
First I suspect most fish don't live Normal life spans particularly tropical fish from predation to ambient bacterial levels at warmer temps.etc .etc. bettas have all these things going against longevity but I personally had one live over 5 years. but average was 2/3 years Had a goldfish that was 22 when swept away by a flood . while all the other ones added lived no more than 5 to the same pool.
Curious as to how you're keeping your Betta?? To me it seems logical that they would live longer in larger, cleaner environments though I'll admit that I have seen them live in pint jars for comparable lengths of time.
From working in a pet shop I'd say you're WAY above average!! Congrats!!! gary