Hi, Tramd and welcome to ECF!
Please, whatever you do, never ever let your carto get dry!!! They need to be topped off early and often in order to keep them in good working condition! Once they taste dry--or even worse, burnt--they are toast and need to be discarded.
The math is simple: one cartomizer contains about 1 ml of eliquid--or 20-25 drops. One drop of eliquid will give you 4-6 puffs, depending on the Wattage and on your vaping style. The best case scenario: 25 (drops) x 6 (puffs) + 150 puffs. And that means you'll be vaping your carto dry--which will cause burning of the filler. So, for all practical puposes, you should never go beyond 100 puffs, more or less, before topping off.
Get some eliquids compatible with the flavors that come in your prefilleds and keep your cartos moist at all times.
A good cartomizer can successfully process 6-10 ml of ejuice, again, depending on the circumstances (Wattage, VG/PG ratio, how much flavoring/coloring/sweetener your juice contains etc.). Once the flavor and vapor production begin to diminish, even from a freshly refilled carto, it's time to throw it away. They are disposable, so unless you absolutely have to reuse them--throw them away. Cleaning never brings very good results as the coil gets covered with gunk that is pretty much impossible to dissolve without dry burning--and one CAN NOT dry burn cartomizers.
Happy vaping!