Putting aside the fact that it's different from person to person (which is the case with all chemicals and drugs), I'm guessing the variables are:
1. Nicotine strength of the juice
2. PG/VG ratio
3. The mod and atomizer settings. This has too many sub-variables. Wattage, resistance, TC, MTL/DL, ...
4. The number of puffs, puffs' duration and the interval between them
5. The flavor's chemical properties
6. The amount of juice vaped. However, this one is not independent from 4.
7....?
This is a too complex challenge to tackle even for someone who has the academic background. But I think it is possible to exchange thoughts about the impact of each variable. For example, is VG/PG ratio significant? What about the flavor?