My 2 biggest Killis are pushing a year and going very strong they are on fun diet of cricket blood worm a bit of flake, brine and ghost shrimp the female is just over 2 inches
its usually around a year....some can live 2 yrs and according to a recent article in a mag i read....some species can live upto 5 yrs.....so they must be semi-killifish
In natural habitat many killies will live for only a year but this is because their water dries up and they die but the eggs are protected in soft/damp earth and hatch when next rains come... - so in the aquarium they are not killed by their environment.
There are also some species that arn't annuals and live for several years - so it depends on which you have...