There are many good sources that tell of nicotine content in regular cigarettes.. here.. Let Me Google That For You:
Let me google that for you ( Click the link to go to your ready made search )
The Wikipedia article is pretty good but it only slightly touches on your question:
Electronic cigarette - Wikipedia, the free encyclopedia
You can find a lot of info here about how much nicotine we are actually getting:
Medical Research You'll have to browse through the threads.
We don't know exactly but one way to tell is you can get a close approximation by listening to your body. You know how you feel when you've had enough nicotine from years of smoking cigarettes. Most people try to achieve that and they will know their limit on nicotine intake.
Caffeine and nicotine are both central nervous system stimulants. Nicotine is not cancer causing. The harm cigarettes do don't come from the nicotine but from the tobacco smoke that is burning. Burning tobacco has all kinds of bad stuff in it like:
Arsenic: used in rat poison
Methane: a component of rocket fuel
Ammonia: found in floor cleaner
Cadmium: used in
batteries
Carbon Monoxide: part of car exhaust
Formaldehyde: used to preserve body tissue
Butane: lighter fluid
Hydrogen Cyanide: the poison used in gas chambers
Over 4000 chemicals in all plus 599 additives the tobacco industry adds that are toxic. That's why cigarette smoke is bad for you. Electronic cigarettes only contain nicotine which has proven to be non cancer causing, does not cause heart of lung disease.
This article has some good info on this:
Is tobacco safer than electronic cigarettes? - by Elaine Keller - Helium As well as lots of info at CASSA.org
CASAA | The Consumer Advocates for Smoke-Free Alternatives Association
I suggest you read through everything else then read the medical-research forum last.
Here is a doctor stating that nicotine is as safe as caffeine
Prepared Witness Testimony: Rodu, Brad - 3rd paragraph.