For example, a person who smoked less than half a pack a day may need 36mg liquid to avoid cravings (and show absolutely no signs of nicotine OD while using it); a person who was on 2 PAD may be happy with 18mg. There is no comparison possible because the end result required is satisfaction / lack of cravings in an individual - and individuals vary so much in their tolerance to / requirement for a given substance that there is no 'scale' that can be devised.
There are some useful things to keep in mind here, though:
Using an e-cig is different to smoking a tobacco cig. Newbies have such bad technique they may only get a fraction of the nicotine that an experienced vaper would get from basically the same set-up.
It is difficult to make your e-liquid stronger, but very easy to make it weaker.
Because of this it is a good idea to get some high-strength 36mg liquid with your first order. If, while using the 24mg commonly supplied with Starter Kits, after several days you still experience strong cravings, you can try the 36mg liquid. On the other hand, if your main liquid is too strong, you can add VG
(available at any pharmacy) to it, to reduce the strength to whatever is required. Another way to do this is to use 'zero-nic doubler liquid' from your vendor - this is flavored base liquid with no nicotine that is specifically made for reducing the strength with.
If you have some 24mg strength liquid, and add an equal amount of VG
or doubler liquid to it, you then have 12mg strength liquid, double the original quantity. Adding VG
will reduce the strength of the flavor, however. Adding 'doubler liquid' decreases the nicotine strength but maintains the flavor level.