I am just starting to use cartos too, and still waiting for my little chucks so I feel for ya
I have been reading up on them, and just got my first 3 packs from different companies yesterday and today. So far I have been able to load about 20 drops in on the first load (trying not to flood them) waited about half an hour after filling to let the liquid soak through the filler to the atty, and they are working great. From what I have been reading, in order to get the most life out of these, you should not let them run too dry, as soon as they start to show change in flavor or vapor production, top them off with 10 or so drops (after the initial load on a dry carto) I have already flooded one carto topping it off too soon after loading it, and it was gurgling and not making as much vapor so I blew the excess liquid out. The point is to try to avoid burning the filler inside by keep it wet, so you can keep refilling it over and over. It seems the common agreed upon time to use then clean is after 3 or 4 refills (that may vary from person to person) then clean with either high proof alcohol, or just run under hot tap water. There are plenty of youtube vids to watch about cleaning and using them. Once you find they are not working so well, you are also supposed to be able to take out the filler and drip into them (from what I understand) so you don't have to toss them away when they start getting used up unless that is what you are into. I have not had mine long enough to know how long a single load will last, but it's definitely lasting much longer than what I was getting from my 510 cart, having to top that off every few puffs.
So far I have tried the 5 pk blanks from:
Vaprokings.com: works just fine so far, cap on top comes off easy. (5pk $14.20 shipped)
Altsmoke: also works the same as vaporkings, easy to remove the top.(5pk $13.94 shipped)
Vapor Nine: has the "new" carto model, they work great so far but the cap is nearly impossible to pry up with out mangling it so I have not opened it up to see inside yet.(5pk $16.55 shipped)
again I am a carto newbie, and just know what I have read about them here in the forum, watched on youtube and the 24 hours or so of using them so far. I also only vape VG so I don't get much of a throat hit, but the flavor is great so far. From what I read, if they start to run dry, you will get a burned taste. Some people are having issues with the filler getting "burned" but I don't know if this is a random problem, or a user problem, etc... I plan to keep mine wet, and hope they don't burn up. Good luck, and let us know what you end up using!