You get something like 4 or 5 puffs per drop. I read it on one of the stickies or an article in the Infozone, so it has to be true.
Assuming that those puff counts are correct, and assuming a nicotine absorption rate of 27% percent (the average of 15% and 40%, the min and max I've seen cited), that means that at 24mg, 19 puffs = 1 full-flavor cigarette (1mg nicotine), and at 18mg, 19 puffs = 1 full-flavor cigarette.
That's just how the math works out. Based on my personal experience, those puff counts strike me as a bit high, which means that nicotine absorption is likely higher than 27.5%. Adjusting the absorption rate to 35% means 15 puffs at 24mg and 21 puffs at 18mg. That strikes me as closer to my experience (though, admittedly, my experience with cigarettes -- vast though it may be -- doesn't yield a quantifiable comparison).