Well all I can say is I get mine from LiquidXpress. I don't know who makes theirs but I have purchased 60 or more and never had a single one DOA. I had one that seemed like it had an air leak so vapor was week, but I still used it.
I buy them pre-filled, they are the same price as blanks so its a no brainer. Why pass up on free juice? I get them in Grape, Traditional Tobacco, and Taboo (RY4). Then I keep them wet with the same juice, I have all three in 30ml bottles.
If you go even one hit with them too dry they are done. Once you get a slight burnt filler taste its too late. That makes it harder to tell for me with the tobacco flavor ones so I tend to top them off more often.
To top off I drip about 8-10 drops around the top of the filler, put the condoms back on and let sit for a few hours or more.
I never top off and go right back to using it, it seems to take a long time for the juice to distribute fully and you will burn them IMO.
Next, and this might be a big one ..... I DO NOT CLEAN THEM !! I tried cleaning a few when I first started using them and all it does is shorten their life IMO.
Honestly I have never tried or cared how long they last. I just know they last a long time for how much they cost. A box of 5 is 8 bucks from them and I spent $9.xx a day on cigs. I just cant get worked up about them even if I had to use 2 per day and chuck them both a the end of the day.