As there isn’t any kind of indicator on e-cigarettes to tell you when to change the cartridge, you have to develop your own system. Most people I’ve spoken with say you’ll generally need to change your cartridge from once per day to once every other day. It really depends on how much you smoke, but in general, expect to change every 24-48 hours.
Personally, I find that I notice less vapor coming from the e-cig and the hit of nicotine growing continually weaker. Before e-cigs, I smoked about a pack a day, and I’m needing to change my cartridge daily. Seeing that 25 cartridges cost me $25 (from BluCigs) I’m paying a buck a day to smoke rather than $6 or more for Marlboro Lights. Not a bad deal.
I’ve also read that you should ensure your battery is fully charged if you think it is too early to change your cartridge but the hit is weaker.