what would be a safe amount to vape each day on a 18mg nicotine and 50/50 pg/vg mix ?
There's so many variables that it's impossible to answer that question.
18mg in my KR808D1 would produce massive amounts of vapour and lead to an overdose if I used it too much. In other e-cigs it wasn't enough.
Also depends on juice supplier. And how much you smoked.
As soon as you feel ill, notice your hands start to shake, or feel anxious and hyperactive, or experience insomnia (or any nic OD symptoms), then you should lay off it. If it's a regular occurrence you should consider stepping down to a lower nic level. Listen to your own body rather than people on the internet. I think most of the people on here who write about all day vaping with very high nic levels are mental, but if it works for them it's none of my business.
I'm on 6mg and an ex 20 a day smoker, I never have any cravings for a cigarette.