I started on 24mg/mL in order to quit smoking. I wanted to make sure that if I encountered difficulty breaking my dependence on cigarettes, it wouldn't be because I wasn't getting enough nicotine from my electric juice.
While I was in my "Halo phase", I tried stepping down to 18mg/mL, but that was apparently too big of a step for me as I just wasn't getting enough nicotine to satisfy my body/mind, almost no matter how much I vaped. I was having several nicotine fits per day. So I took my 24mg juice and my 18mg juice and mixed them 50/50, making 21mg juice. That worked perfectly for me.
Then after a month or so on 21mg, I much more successfully stepped down to 18mg. And 18 is what I still use today. I've been vaping smoke-free for five months and two days. So far, I haven't felt an incentive to step down my nicotine any further.
I know that any chemical that has a physiological effect on the human body (except if taken under the care of a doctor for a particular medical condition) isn't healthy. It isn't harmful, either, based on what I've learned. Whether it's addictive is still being debated by the scientists, although I'm pretty sure that it's nicotine withdrawal that I'm feeling if I'm not able to vape for several waking hours for whatever reason.
So being fairly healthy other than the nerve damage in my neck and left shoulder/arm/hand, the risk of long-term consumption of a moderate amount of nicotine should be quite small for me. Plus, the nicotine helps mitigate muscle spasms a bit, which I welcome. So I foresee that I'll be using 18mg nicotine for a while.
I may step down to 0mg for the practical reason eventually - so I can go without vaping however long I may need or want and not have to worry about nicotine withdrawals, but that's only IF my condition improves someday and I don't need the muscle relaxing effects so much.
At this point I doubt that I'll ever quit vaping, however. I just love doing it too much, and I love how important it is to society.