I haven't kept accurate track, but my guess is that I have filled this set of 30 cartos (two have been replaced from my stock of fresh ones because the originals quit working - burnt out or broken heaters) at least 8-10 times. Note that this is without cleaning of any kind. Only on the last few fills do some of them seem less-than-able to take a full charge of 20 drops.
I don't think it's caramelized sugar (at least I hope it isn't). I avoid all juices that are described as, or even hinted at, being sweet. "Cordial" and "cobbler" are signs to me that I wouldn't like them. It may be that ALL juices have sugar, but I hope that this is not so. With the juices I use, a drop on the tongue (as when a carto is a bit juicy and spits a little liquid) does not taste sweet to me, but any sweetness may be masked by the strong taste of the raw juice.
I also avoid juices that suggest any oiliness: "chocolate", "buttery", "cappuccino" and the like. My feeling is (besides taste) that sugar and oil are likely to result in "dirty" cartos after repeated fills and uses.
I use V4L Sour Cherry (V4L is currently out-of-stock - sob), Sour Green Apple, and one of the Menthol juices which has no "sweet" in its description - all in 36mg strength. I use red, green, and blue cartos to keep track of which carto has which flavor. It also helps me rotate them (RGB, RGB, RGB, ...) to give them time to cool off between uses. Otherwise they get unpleasantly hot on the lips. (90F is hot, but OK; over 100F is unpleasant. I just checked with an IR thermometer.)
A set of 3 flavor cartos lasts me 4 or 5 hours before running dry (a bit less if I'm really hitting them) so, a little over 80 minutes per carto. (About 180 drops of 36mg per day.) I switch to fresh XL batteries each time I change cartos. (This requires 9 batteries and 6 chargers.)
--
Oscar Wilde once said: "Today I spent [a short time] putting a comma in; I spent all day taking it back out." Perhaps parentheses are similar. (I totally mis-remembered this quote. He actually said: I was working on the proof of one of my poems all the morning, and took out a comma. In the afternoon I put it back again. )