Modes of thinking differ on this. The first thing to finding out how much you're taking in per day is to figure out how many ml of juice you are using a day to stave off cravings.
Some say you absorb all the nic from a puff, some studies indicate you only absorb 10% to 33%. So you need to find out how many ml you take in per day, multiply that by the amount of mg/ml (labeled on your juice), then find 10%,33%, or 100% of that number and figure out which is the most realistic.
I'm under the belief that you absorb about 20 -23% of the nic content and here's why:
Crunching the numbers (which I could be wrong on the method but hear me out) at 10% I'd be smoking the equivalent to 14.4 analogs a day. If that were the case I'd have given up the E-cig and gone back to analogs by now.
At 33% the number is around 47 analogs a day. As a previous pack and a half smoker I would have felt sick smoking 17 extra cigarettes a day.
20 - 23% gives about the number that I used to smoke in analogs and seems to be how I feel. I'm not sick or getting headaches, nor am I edgy like I'm getting too low a dose. My cravings are being met and maintained as they were prior to quitting analogs.
At 100%, well that's just absurd. I would have died already...twice over.
Breaking it down is like this. I realized it took 4ml of my juice per day to manage my cravings. I vape 36mg juice.
36mg/ml x 4ml = 144 (that's your 100% total)
144 x .20 = 28.8 (about the same as I used to)
This is all my own observational empirical evidence and not hard lab tested fact. It's how I guage my intake. I believe my body is giving me the proper response to the amount of nicotine it has been used to for over 2 decades. As we all know a smoker's body will tell them hands down if they are getting too little or too much nicotine.