I never clean my cartos, but I don't use them for weeks at a time, either. They're two bucks. When they get nasty, I toss them. 
Creeps me out a little to hear of the same 2 cartos being used for several weeks without cleaning. Doesn't sound sanitary. Like drinking from the same cup for a month without ever rinsing it out. Yuck!

I find that the dark juices kill carts faster. If you want them to last longer, use clear juice. I prefer the dark liquids, so I just use the cart for a day or two and then soak overnight in pga and run through a couple cycles in an ultrasonic jewelry cleaner. This seems to revive them for another go.