In Star Trek: The Next Generation, the bartender in Ten Forward is the enigmatic Guinan, a member of a race called the El-Aurian. While it is generally known that her race is long lived, in the episode Times Arrow it is shown that Guinan visited Earth in 1891, at which time she looked relatively the same age as she did in the 24th century. This would have made her at least 500 years old by the 24th century. So how long do the El-Aurian actually live?
There doesn't appear to be any specific information regarding the maximum or average lifespan of an El-Aurian. Of the known El-Aurians (some never appearing onscreen), the oldest appears to be Guinan's father:
This would place his youngest possible age (if born in 1600) at 769. Assuming Guinan's father represents an average El-Aurian, I would say an El-Aurian lifespan is, at minimum, almost 800 years.