I comfortably vaped 18 or 24mg on old school attys for years. When I finally joined the industrial revolution I dropped down to 6mg. 6mg is still stiff for me as much as I vape, so I've gone to 3mg and am happy there. Your body will tell you, there's no mistaking nicotine sickness.
I smoke cigars and take nasal snuff and never feel nicotine so you'd pretty much think I'd be immune, but I'm vaping 3mg now. YMMV