One filled cartomizer should last around 1-2 days, and you'll know when it's getting empty as you may get a slightly burnt taste, but usually you'll find that you're not getting as much vapour.
At this point, you can dispose of the cartomizer and use a new pre-filled one, or you can refill your current one using your favourite vape juice or eliquid. You can find out how to fill a cartomizer here.
If you do refill your cartomizer, you can expect to get around 5 refills before needing another one. You'll know your cartomizer is completely finished as the burnt flavour will persist or you'll be getting little to no vapour at all.