Check this out:
http://www.e-cigarette-forum.com/forum/ecf-library/211345-new-users-nicotine-strength-questions.html#post3799856. It says it all. As others have said, it's a totally individual thing.
I never liked light cigs and smoked mostly Camel filters (and occasionaly unfiltered Camels and unfiltered Lucky Strikes [old school, baby! lol]) and eventually full-flavored generics as prices rose. I smoked cigs for 17 years 1.5-2 PAD), then finally switched to a pipe 9 years ago. I always smoked unfiltered pipes and mostly Prince Albert tobacco. This makes for a combo about as potent as unfiltered cigs as far as I could tell (I inhaled the pipe most of the time), yet when I started vaping about 1.5 months ago, I tried out 18mg and 11mg and settled on 18. I do vape more often than I smoked but not immensely so.
My point is that you'd think I'd have needed at least 24mg (if not 36mg) to kick the habit after 26 combined years of smoking some pretty heavy duty tobacco, but I found 18 quelled my need for tobacco just fine (not enough TH with 11mg for my liking). I smoked the pipe along with vaping for the first couple weeks, but found vaping much more pleasant so eliminated tobacco altogether 3 weeks ago (going on 4 weeks!) and haven't had the slightest urge to go back to the evil vile leaf (just noticed evil and vile are made up of the same letters. Freaky!)

.
All you can do is try several levels and see which one works for
you. You can try starting with 11 and if you start getting antsy up it to 18, etc. I'd buy bottles of several strengths (say, 11, 18 and 24 [if you want]) all at the same time, so you're not stuck waiting if one proves not to be strong enough.