Im a newbie also, but here is my experience with cartos
I bought empty cartos and filled em myself. I just had to refill the 1st one (after 2 days), I noticed a little harsher taste. I have the wyatt earp and ry4 e-juice which has kind of a sweet flavor, so it was a little easier for me to tell the difference in taste. The first time I filled it, it took about 30 drops (I wasnt counting) now when I refilled it, it took 17 drops (I counted) before I saw a little pool coming out of the top (mouth part, as I drip in from the batt connect side). I believe it only took 17 drops because the carto filler was still saturated (which is good because you dont want to vape em dry).