From reading about the experiences of so many, I think the only rule is... there is no rule. Some people can (and need to) chain-vape 36mg all day and night. Unlike tector, I love the throat and lung sensation of high-nic, but reserve it for early morning vaping, and drop down to 12 or even 8 for the rest of the day or I'll start feeling a bit ill. (Compared to many here, I was only a heavy-ish smoker, like 1.5ppd Marlboro full-flavor or Mediums at peak, for around 40 years.) But as you can see from mutt's posting, heavy smoking doesn't always indicate high-nic vaping.
What I suggest is that you get your nic level figured out even before you experiment a whole lot with flavors. Anything you buy, buy several nic levels of it, including 0mg. Get a pipette or graduated cylinder and empty bottles to mix down to the level that works for you. (e.g., 2 parts 36mg to 1 part 0mg = 24mg). That way, the 36mg won't go to waste if it's more nicotine than agrees with you. There's just no predicting...
I agree with what everybody has said about models. Can't go you wrong with a 510, and the Ego is the same thing with dramatically more battery life and, I think, a bit more warmth and oomph.
Happy vaping!