A very rough estimate says that a traditional tobacco cigarette, as I remember, contains about 7mg of nicotine, about 1mg of which is absorbed into the body when you smoke it. So if you're taking in 7ml x 3mg/ml = 21mg of nicotine per day, that's roughly (very roughly) equal to about a pack-a-day habit. That's assuming you absorb 100% of the nicotine in your e-liquid, on which I'm not sure there have been any conclusive studies yet.