That entirely depends on what species you get. Some species of killies only live a year (they live in ponds that dry up once a year) while others live for a normal lifespan. They are some very interesting fish.
You should be able to find information on the particular kind of Killie on this forum -
http://www.aka.org/aka/modules/content/index.php?id=2
You might even be able to find a few pictures to help you determine the kind of Killie you want!
I'd do alot of research into Killie terms (like annual) and research the particular kind of fish you want. Good luck!