I don't know if there is a specific time that the cartomizer will last. I've read in these forums that it could be anywhere from 2 weeks to 1 year, or possibly even more. Avoiding dry hits will help prolong it's life. Think of it as setting a pot on a stove. As long as you keep water in it the pot can stay there for a very long time boiling water but if you let the water boil dry it will burn the pot up pretty quickly.
You'll just have to use the ones you have and once they stop producing vapor then they need to be replaced or rebuilt.