A lot of factors play into the length of life on a cartomizer.
1) Flavor of juice. Sweeter and darker liquids tend to clog cartomizer and clearomizer coils a lot faster than lighter or less sweeter juices. Sweeter juices and those with a lot of flavoring, tend to get gooey or gummy a lot faster, building up gunk, carbon, and residue quicker, which chokes and coats the coil a lot faster. Darker liquids like coffees and such, same principle.
2) Higher VG and thicker juices fall prey quickly to the choke and coating symptoms due to the base that is used. A 70PG/30VG to 100PG/0VG juices are not going to build residue as fast as a 50PG/50VG blend, which is less than a 30PG/70VG or 0PG/100VG blends will do
I do not know how your friends maintain their cartos, but there are a couple of things you can do that might and might not work to extend your cartos life. General cleaning as far as cartos go, about the only one that can help is to boil your cartos in water, let cool, then blow out and let dry upside down on a towel or paper towel. This can extend the carto life by about a week, possibly more, but generally a week of service, the moment you start feeling a drop in perfomance and air flow, the damage is already done, and nothing can really help, there is a point when you start to notice the point air flow is starting to get restricted, the carto needs boiling to clean or soak is say vodka or everclear to clean up that gunk, go past that point, continued use from then on is compounded upon.