Have you ever thought about switching to the bottom coil phoenix from gotvapes? It looks like a cartomizer and is the same shape, however it doesnt have fill. They dont leak a drop and have a great vape. They last a very long time and can be easily cleaned.
If you do want to stay with a cartomzier with fill, I suggest the cartomizers that smokeless image sells. They are horizontal bottom coil. They dont burn nearly as easily as other companies. I have a video on youtube that actually teaches how to turn one into a drip tester. In the video, I show the difference between the smokeless image and other cartomizers by showing you the inside.
How to make your own Juice tester with an old cartomizer - YouTube
I personally would not bother trying to boil them. Its hit or miss as to whether they will work after. Try using similar flavors in them if you really want to use a different juice in one. Or better yet, take a look at the phoenix. One of my videos discusses that too.
As far as how long a carto lasts, that depends on your juice. High PG and dark juices will make it go bad faster. The thicker the juice, the more it "gunks" up. With the smokeless image one, I was able to refill them over and over. They lasted me about 1 month using 70vg/30pg juice