The simple answer is nobody really knows. Lots of conjecture and semi-scientific reasoning, but little real evidence that I know of - although I would be delighted to be educated with some studies if anyone has links to them.
As to your original point, 15ml a day is a lot. You might want to consider some of the suggestions above about your delivery device and your nicotine levels.
I just completed 2 years with no cigarettes and I have gone from 24mg to 12mg where I am at right now. 6mg would just not cut it for me no matter how often I vaped.
I think Lessifer hit the nail on the head - it's your journey, only you can decide what is right for you.