I started on 18, which is fine if, like me, you start out by vaping it the way you smoke - smoke 4-6 mins, and take a break between vapings. However, you will quickly notice you can vape a lot more than you can smoke. That's why people move down in Nic strength over time - vaping is really nothing like smoking, you can basically vape continuously and not feel sick (except from nicotine poisoning.) I smoked a pack and quarter a day, alternating between Camel 99 filters and Came 99 lights. So that's a pretty heavy habit, I think.
Frankly, I'm learning this the hard way: I got some samples of 24 mg strength and they are so tasty I am having a hard time not puffing on them. So I'm a little woozy tonight, lol!
So if you can stick to vaping 20 or so times a today with breaks between, high MGs are ok, if you want to chain vape, lower it down, fast, to somewhere between 8-12 mg.