I'm a newbie on this, but in my opinion there are so many variables that it' really hard to say. I quit cigarettes a few years ago after smoking for roughly 10 years. I was around a pack a day smoker. When I quit smoking I simply took up another habit which was Camel Snus (a mild dip). Well I bought my e-cig around Thanksgiving of last year and I've been Snus free ever since. At first I thought I'd need 24mg because of the fact that dip in general delivers far more nicotine that cigarettes do, so I went 24mg and figured out real quick that it was WAY too much for me. I dropped back to 12mg, and that was much better, however, I find myself using my e-cig quite often just as something to do (the reason for most habits).
And my initial goal of this adventure was to cut nicotine completely anyway, so I went ahead and dropped down to 6mg, then 3mg. Now I'm on 3mg, and I have one 0mg that I can splash a couple drops of higher nicotine into (same flavor) if I need to. I must say, though, that 6mg and lower there really isn't much throat hit at all (which I'm not too concerned about). I would suggest someone just starting out to start at around 12mg or so as a sample, then decide for themselves whether they need to go up or down based on their own habits, and preferences.
Now my neighbor on the other hand, he smoked roughly a pack a day and switched over to e-cig when I did and he likes 18mg. He likes the throat hit. I don't think he's tried anything lower than 18mg, but he said he's content with 18mg and that he probably won't. Another friend of mine went from dipping Grizzly (very high nicotine content dip) to an e-cig. He started at 18mg (which I thought would be either about right for him or tad too high, and he actually moved up to 24mg, stating that 18mg just wasn't quite enough for him.
So as you can see, it's really been all over the place for my recommendations for others. Maybe someone with more experience will chime in here and tell how to make a more accurate recommendation as a starting point for someone new to
vaping, based on what product they currently use, how much, and how often.