So far I've come to three variables that effect nicotine delivery. The first is the juice, how much is in solution for your juice. Obviously more milligrams will be more nicotine to your membranes. Second is equipment. If you're a dripper, expect to get less nicotine out of your juice, but tanks (and especially Kayfuns) are more efficient at delivering nicotine to you. Third is habit, are you a chain vaper, do you hit it once an hour, etc.
General observations, while cigarettes are exceptionally good at nicotine delivery, I actually get much more nicotine when vaping. It's cumulative, 20 minutes of chain vaping (5ml of 6mg juice=30mg of nicotine) is 15 analogs (I was using American Spirit Red rollies, shown to have 2mg per cigarette). Because the efficacy of my equipment is lower than an analog, I estimate %75 absorbed (totally optimistic), so the number drops to about 24mg, or 12 analogs in 20 minutes.
My math is quite probably wrong, but that's what it feels like. 40mg could be considered lethal, but since I'm Scottish, it just kind of tickles.