There are a few signs to know when to change your carto.
1. You will notice that the draw becomes very tight.
2. Performance diminishes.
3. Ultimately, they will taste burnt. Although, I recommend changing before that time. There is a long thread in the cartomizer issue section of the forum explaining the dangers of a burnt cartomizer. Might be worth checking out.
Generally, I only use cartomizers to about 15ml.
It's really not worth trying to extend the life of a cartomizer. Although, if you really want to clean them.
1. Then you can boil them for several minutes until you can smell the
e-liquid.
2. Drain and repeat until you can't smell anything.
3. Let them dry from anywhere from 48 hrs or two weeks. Depending on the weather in your area. Some do bake them in the oven but, I've never tried that before...so, I'm not gonna comment on that.