I love using cartomizers, easy to use and no fuddling required!
I use a 510 type, using the same liquid and using the carto regularly instead of leaving it sit, I get two weeks of good vaping, week three, the taste seems to change. I will admit, I've gone four weeks easy on the same carto, though I do try to just toss and replace around 2-3 weeks.
I push the limits, two weeks is probably a good rule of thumb and just clean your drip tip on occassion. Every now and then I do get a bad carto, I just toss it. If you have troubles with them on a regular basis, try a different vendor.
My trick for longevity: I fill the carto up good thru the top, I throw away the little white stopper and use a drip tip, I top it off very much like those who drip with attys do, I never counted the inhales, sometimes I add a drop or two after a half dozen hits, lately I've been going a bit longer, I am guessing about 12 strong hits but adding about five-six drops. Note, this varies depending on what type of battery you use, I use 6V so you could easily double that if you are using a lower voltage battery.
Basically the very second you feel it getting just a fuzz dry, add some juice-just drip it in thru the tip. When I first started doing this I always overfilled and had to remove and blow out the excess into a paper towel, use a bit more juice but one eventually gets it down to a science.
If I let a used carto sit and kind of dry out for days at a time they just don't seem to last as long.
The type/flavor of liquid you use will definitely come into play, some just don't seem to taste so great after about a week while others seem to be pretty consistant for weeks on end. I know I push the limits with these but for me, it works.
Also, figure a carto cost you 1.40-1.75 so even if you go a week, that is still pretty cheap compared to a daily cup of coffee or worse, a pack of cigs!