It has proven very difficult to calculate how much ecig users consume per day since there are no easily-identifiable data points that can be used. The least useful method would be to try to quantify the amount by using puffs, or puffs per day, as I have never heard of anyone who would be able to accurately answer that. There is however a new ecig model with an electronics package that does measure such data and can be connected to a computer to download the information. Perhaps hardware of this type could be used in clinical trials.
However, this information might not be of much use since an e-cigarette doesn't work if a puff length is employed that is the same as is used for a tobacco cigarette - the puff needs to be at least twice as long, for several different reasons (otherwise little or no nicotine is delivered, as shown by multiple clinical trials such as Vansickel Eissenberg and Bullen et al).
There are two possible ways of measuring consumption that have some chance of reasonable accuracy: refill liquid consumption (for those who employ bottled liquid refilling) or carto consumption (for those who buy pre-filled cartos, or who refill their own and therefore know the refill times per day, and especially for users of the regular size cigarette clone models). The last usergroup is certainly in the majority - but are very hard to reach online, as most tend to be unaware of the existence of an online e-cigarette community (otherwise they would (a) participate and (b) upgrade) - neither of which occur in large numbers. In other words the online community are perhaps 5% of the total number of ecig users.
Most bottled liquid users will know either their daily, weekly or monthly consumption. The average is about 3ml per day. 1ml of liquid is approximately equal to 6 cigarettes. 1 cigarette is roughly equivalent to 3 to 4 drops of liquid, in the opinion of most expert users.
The standard cartomizer holds 1ml although not all can be used as some is retained by the filler; perhaps 80% can be used from each pre-filled carto. A smoker who consumed 15 cigarettes per day will typically use 3 cartos per day [1]; each is equivalent to about 6 cigarettes. There are about 22 'drops' per mil. This is a flexible concept since a drop from different droppers or dripper bottles will obviously vary in quantity, but as the averages are important not the precise measurements, a figure of 22 drops per mil is useful.
So if we were to ask user how much liquid they consumed per day, a large proportion would be able to answer that; and if not by direct knowledge then by some form of calculation. Beginners need to be asked how many cartos a day they use. This is an inexact question because although the benchmark carto is the Boge 1ml 35mm x 9mm cartomizer, there are many other sizes and types. All cartomizers ( the 2-piece system) hold more than the cartridges used by atomizer-cartridge (3-piece) systems, which is 0.6ml for the benchmark 510 system. Other systems may contain less.
Determination of refill liquid consumption is an inexact process and may be subject to numerous errors. The best accuracy would be achieved by asking multiple questions, leading to a funnel that increased accuracy. Even this method could not guarantee any particular level of accuracy since some users will know their consumption precisely, and others will simply guess. Otherwise, quantitative measurement reporting is likely to be mostly erroneous.
[1] However, there are plenty of examples of anomalous equivalancies since vaping and smoking are different. While many 15-a-day smokers will indeed consume 3 cartos per day, there are examples of 15/day new users who will consume 1 carto a day or less.
The experience of many (perhaps most) liquid refill users is that they vape 'more' than they smoked. The concept of 'more' is also a flexible one: does it mean more often, or for longer, or with more nicotine, or what exactly.
New users employing pre-filled cartos may sometimes vape less than they smoked. However there is a much higher rate of failure among new users, utilising mini size hardware, than among more experienced users with better hardware that uses liquid refilling.