I don't think there is any way to equate nicotine supply from e-cigs with that from cigarettes as there are too many variables and in any case it eventually becomes a pointless exercise.
Firstly, nicotine obtained from e-cigs is purely that, and nothing else. Obtained from cigarettes it is combined with synergens and other materials that almost certainly affect the way a human body receives it, uses it, reacts to it, metabolizes it, and so on. This can be seen from blood tests on vapers such as the one linked above: none of the figures line up exactly with those obtained from smokers, and some are way off.
We know that the amounts of nicotine obtained are comparable, from the effects experienced and from research such as Dr Eissenberg's most recent tests that showed a 30 or 40ng/ml level which is comparable to that in smokers. But as for the total effect when the other materials in tobacco are considered, or the time needed to obtain X amount of nicotine from a cigarette vs an e-cigarette, it seems to me that trying to equate one with the other - especially if trying to do so by making some sort of statement about the nicotine strength of the liquid needed for an
n-pack a day smoker or a smoker of light or strong cigarettes is a waste of time.
The number of variables is almost infinite: whether using a tier 1, 2 or 3 e-cigarette [1]; the strength of the liquid; the type of liquid and how that affects absorption; the possibility that vapor has more buccal and nasal absorption than within the lungs due to the larger particle size; the differing size of the inhaled volume between different e-cig users; the additives present such as menthol that affect the half-life of nicotine; the tolerance and metabolism differences between users, and so on.
I would smoke a cigarette in 5 minutes, and need to vape for 10 -15 minutes @ 36mg to get the same perceived result. For six months I vaped a lot more than I smoked, and used a 5 volt device a lot, but that has settled down to about 50% more and mostly with a standard large-format model. Someone else would most likely give different numbers.
[1]
A way of classifying e-cigarettes for various purposes