Could you please let me know what you consider the longest lasting cartridge filler? (seem to need to refill mine very quickly)
Short answer: Shredded PTB!
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There are two factors affecting how long a cartridge lasts:
1) (Generally the most important.) How well the material "wicks" the liquid to the atomizer. If the filler doesn't wick well, it won't matter how much the cartridge holds - after using just a bit of the juice in it you'll need to top up.
2) How much liquid you can put into the cartridge. For most materials this isn't a big factor. With polyester fibers (the stuff cartridges come with) or PTB, the filler material takes an insignificant percentage of the space in the cartridge. You'll get well over 90% as much liquid into the cartridge with these fillers in place as you would with an empty cartridge.
Some other factors come into play in this. You don't want a filler which doesn't hold the liquid well enough to avoid flooding the atomizer. And you probably don't want a filler which has a chance of being vaporized itself. (Those polyester fibers can create an awful taste if they stray into the hottest part of the atty, and I doubt that those gasses are healthy.)
The shredded PTB filler takes some practice to get right. I did it quite a few times before I felt I'd perfected it. If you want the ultimate result it is important to understand how it works. You want to end up with a lot of PTB fibers which will surround the atomizer's bridge even after pulling the cartridge out, refilling, and reinserting it numerous times. But the final result is awesome. I can now make a new one in a few minutes for either a 510 or an 801. I can vape over 75% of the cartridge's raw juice capacity with the same vapor output as direct dripping. (If you haven't done that, please just consider it to be the gold standard for the best that wicking can ever be.) I carefully pull these out with tweezers, rinse in water, dry in a paper towel, and re-insert when changing flavors. They last a long time if they're not put through that procedure much. (After a few times through that routine they can become hard to re-shape to hug the atomizer correctly.) I have one which I've left in use with the same flavor for over two weeks now, never pulling or cleaning it. I vape that one for about 1/3 of my day (it is my high-nic unit) and haven't noticed any degradation in performance, so I'd say life expectancy (for full time use with just top-offs, i.e. one flavor and no pull/rinse/reinstall cycle) is at least five days of full time use for shredded PTB without flavor changes.
Hope this helps.