My body tells me when I've had enough, too. I used to smoke a pack-n-a-half of marb lights a day, never felt a buz from it. Started vaping on extra high strength (24-36mg) and was having weird vision issues later in the day (depth perception), zoning out etc. I backed down to zero nic and eventually worked my way back up to 24mg. Now I vape less frequently, 2 or 3 puffs at a time throughout the day, maybe every 20 minutes to hour, but chain vape at night. My tolerance is up so I can vape until slightly nauseous without zoning out or losing depth perception.
If you feel dizzy or nauseous, or find yourself staring off into space, blank faced, you may want to slow down a bit.
You may find that that you can vape yourself into a stooper or nausea and still not feel satisfied enough to leave the smokes alone, this is because of everything else in cigarettes, carbon monoxide for one shuts the body down and greatly assists the calming effect of a smoke. So if you're like me and try to vape until satisfied, you just may find that you can't vape enough nic to feel perfect before getting woozy, and you'd still have to have some will power to quit. even in cases like mine, vapor helps tremendously, I'm smoking 90% less than I was before.