I've just been doing this for a little more than three weeks, and I can't tell very accurately any more because I'm going between several flavors in different tanks (if we call clearomizers or glassomizers "tanks" -- still not sure on that) throughout the day. In the beginning I only had a couple bottles of e-liquid, only one of which I liked much, so I stuck with that and could tell how much I was using: something less than 2ml per day at 12mg nicotine.
Now I'm using mostly 9mg nicotine, and
vaping ... hmm, maybe around 2.5 to 3 ml per day is what I'd guess, on an eGo-type, cycling between three-four flavors during the day. (I have a nascent collection of around 15 different flavors now and two or three big favorites so far, so I'm sampling and mixing, and having a lot of fun with that.) I was a big ol' smoker, but an average or low-average vaper, I guess (though I like vaping a
lot more). I think I would consume a lot more if I were using a much fancier / more efficient, more powerful device, but I'm a) still a total newb, and b) feeling very committed to the feel and aesthetic of the smaller size, which seems to fit just right for me. We'll see how that goes.