When I started vaping, I was using 24, but I had to get some 30 to get off the cigs. Even then, 24mg was my "all day", and before long, I didn't need the 30.
After awhile, I decided I'd try to move down. I got some juice in 18, and I mixed the last of my 24 with the 18 so I had a 30ml bottle of roughly 21mg. I used that as an "in between" step, and when it was done, I went to straight 18 for awhile.
Eventually, I did the same thing moving from 18 to 12 with a 15mg in-between step. And again from 12 to 6. I didn't have much trouble with any of these, and I went a couple of months (which is likely more than needed) between down-steps.
When I had been on 6mg for awhile, I started buying some no nic juice and mixing it half/half with the 6mg so I was vaping at 3mg. I did that for months. Longer than needed, I think. I was really worried about going to 0mg. I'd been hooked on nicotine one way or another for decades, so the idea of not having any worried me. Eventually, one of the guys at a local vape store convinced me that the difference between 3mg and 0 mg was hardly noticeable, and so I tried going with just the 0 nic. I still had some juice with nic available, I just quit using it.
I've been off the nic for I think around 9 months now. I miss the throat hit, but other than that, it hasn't been an issue.
I had thought that if I were nic free, it would be easy to stop vaping, but I still vape almost as much as ever. Maybe not quite as much, but close. On the other hand, all of the things that made me want to quit smoking don't apply to vaping, so I haven't put a lot of effort into quitting vaping. I suspect I could do it if I really tried. I did go 24 hours without vaping recently after some dental work, and it wasn't too bad.
All that being said, nicotine isn't really that bad for you. I may go back to 6mg or something in that range.