keep it in a decent tank, with good filtration and good foods etc, the average is about 5years. Unfortunately there are too many people who will happily put a Betta in a 1 gall or less jar with no filter/heater and wonder why they last a year at most.
IMO, it depends on the betta itself. Some bettas catch deseases quicker than other while other bettas can with stand a heck of alot. My personal bettas have high immune systems IMO. I've heard 0-8 years and have seen bettas 6 years old.