I recommend filling them with a syringe. Its very quick and easy and will cut back on it potentially leaking and killing your battery because you overfilled it.
I originally posted this for someone that asked how to fill Ressurector Cartos, but this ir really for any cartomizer. Just instead of putting 2ml of juice that a Ressurector holds, you will put the amt of juice in a syringe that the cartos you use hold.
Heres my copy and paste.
How to fill a cartomizer with a syringe:
"All you have to do is take the dropper part of the bottle off your juice and fill your syringe with 2ml for a new carto. Take the rubber tip off the carto and carefully insert the needle down the side pushing against the metal. DO NOT go thru the filler because it will move it and you could potentially break a connection and your carto. Slide the needle down the side and go all the way to the bottom. Then slowly start dispensing some juice. Dont dispense it all bc then it won't be evenly distributed. Put alittle in, then take it out and insert the needle in a different spot. I normally do it in 3 spots (in a triangle) I found it distributes the best, but you could just do it in 2, on opposite sides. Make sure the needle is at the bOttom bc then you will not have to wait at all for it to settle. Disperse slowly and you will begin to see the toP of the filler become slushy. It is full then, don't put more. If its a new carto is will def hold 2ml, if your refilling it won't take a lot so pay attention and once it gets alittle slushy stop. While your filling hold a napkin or something under the carto bc if you do it too fast a few drops will come out the battery end. If that happens its only bc you did it too fast and the poly fill didn't absorb quick enough.( that happens to me bc I am impatient and fill it in about 10 sec, so I just hold a paper towel or napkin underneath.) wipe up the drops and put the rubber cap back on the carto. Then put the carto between your hands and rub back and forth spinning the carto quickly. This will insure the liquid is evenly distributed in case you put too much on one side from the syringe.
Now you have a perfectly filled ressurector that is ready to be used right away!"
As far as how long a carto wil last, it all depend on what kind of juice your using. Higher VG juices are thicker and will clog cartos faster. Juices that are darker also will kill them faster. The thing I found that kill them the quickest though is using juices that contain sediments. They generally kill a carto fairly quickly. I use the Ressurector cartos and they last normally about 2 weeks for me. I normally use juices that are 80%PG/20%VG. I do use one juice though that has sediments in in and normally after about a week of use they become restricted and I throw it out.
Hope this helps!