How many minutes should I smoke my e-cig?

When I usually smoke my cigarettes (Marlboro 100's), I don't really count how long I've been smoking. Since the e-cig isn't like a traditional cigarette how long do you recommend I smoke? Also, I'm thinking about buying my e-cig from [a href="http://www.electroniccigarettesinc.com]" rel="nofollow"http://www.electroniccigarettesinc.com]/a I'm thinking of buying the "Vapor-King" brand. Do you think this is a safe website to buy from? Any feedback from people who currently use this brand and/or bought from this site?