Two long-term smokers who turned me onto this recommended 24 mikes per mil, which is what I'm currently using 3 weeks in. I can't vape ALL day long, but I tend to vape continuously in the evening when I get home. No ill effects for me, but as always YMMV. Trust your body on this one - if you feel like you should stop, then stop.
This happens when I go visit one of the two aforementioned friends: He has dozens of flavors and mixes, and we "play" vaping this and that and comparing notes. After a couple hours, it's too much and I have to quit.
On my own, I just vape when I feel the need, and it works out OK. Trust yourself, and don't over-analyze the content of nicotine on the label of your favorite flavor.
Inspector 2211