I looked this up last night myself, this is what I found. Again I did not write this.
Again this varies but most people use between 1 and 5ml per day. I suggest you work on 2ml a day as a basic requirement.
To work the exact comparison the following may be used.
1 box of 20 cigarettes equals 20mg nicotine (number x strength)
1ml of 18mg e-liquid equals 18mg nicotine (amount x strength)
Each drop of e-liquid will last for approximately 7 puffs, so 2 drops is roughly equal to 1 cigarette (14 puffs).
There are approximately 20 drops per ml of e-liquid.
The nicotine strength of cigarettes is the amount actually absorbed by the body.
The nicotine strength of e-liquid is the total amount of nicotine per ml of liquid.
The absorption rate of nicotine inhaled from e-liquid is still in debate (some say its as low as 10%) but 50% is generally used as a guide figure.
So 40 drops (or 2ml) of 18mg e-liquid is about the same as 1 pack of twenty cigarettes. (5ml of 18mg = 50 cigarettes. 30ml of 18mg = 300 cigarettes)
Best part of vaping is not caring about only having a few cigs left in a pack