I run 18 - 20 ml consistently, change a carto whenever the taste changes even faintly. I might well get Vapoor's numbers, this is for my own specific DIY mix, high flavor organic, at 8 - 10 watts. Darker/thicker eliquids can definitely have an impact. Carto consistency/QC is also quite variable, I stick with one vendor just because the product they sell is built to their specs.
Initial priming is to me something where it's either done right or the carto is toast -- if I get a burnt taste, insufficient initial soak, I toss it, since once it's bad it doesn't come back to life.