When the draw becomes too tight and you don't get any vapor/hit from it is a good indication. Usually, when it starts going out you'll notice that it's not working as well as it did before. Some will just quit while others will slowly dwindle in performance. As for what I've experienced, most of mine lasts several weeks to a month or more depending (if it's not a dud). Sorry, I know that's not much help but it depends on the part itself.
Asking questions is good. That's how we learn.