Don't believe that for a minute. A lot of these companies, especially those in convenience stores and kiosks, claim one of their cartridges is equivalent to a pack of cigarettes. The kiosk where I bought my first kit claimed it was closer to 1.5 packs. In reality, it's not even close.
swedishfish is correct in the average being 3-5 ml per day. I run between 2-4, depending on what I'm vaping on. Also, just because a carto can theoretically hold ~1 ml of liquid, you're not vaping all of it. There is still quite a bit of liquid left well after it looks and tastes dry. That's why most of us who have done this for a while forgo prefilled carts altogether and get blanks. That way, we fill them ourselves and can top them off throughout the day as needed.
And another point: the consensus is that one puff of vapor delivers about 1/10th the nicotine as a puff from a cigarette. So it takes about 10 drags from an e-cig to equate to one drag from a cigarette. So to get your nicotine levels up to your pre-vaping levels (and, therefore, keeping yourself satisfied and away from cigs), you're going to naturally vape more. Just remember that vapor doesn't have all of the harmful chemicals (tar, ash, carbon monoxide) found in cigarette smoke.
So my advice: vape away with no guilt.