The numbers I found which were only good as of 2003 said that the average number of cigarettes smoked per day is about 15 for white males (about 9 for black males) over the age of 26. Slightly less than 20% smoke more than a pack a day.
Source: http://www.oas.samhsa.gov/2k3/cigs/cigs.ht...

But the actual numbers in no way affect the rest of your argument which is quite true. Pot smokers smoke until they've reached the desired effect, then they stop. Even for overall heavy cannabis smokers, the overall effect is that they inhale far less smoke than even moderate cigarette smokers. Further, they're not inhaling the plethora of substances added to cigarettes. And, even though marijuana smoke contains many of the same carcinogens as cigarette smoke, marijuana smoke has also been found to have a slightly protective effect against lung cancer. Last but not least, the largest longitudinal study to date that looked at actual marijuana smokers concluded that even moderate to heavy use of cannabis does not lead to lung cancer.

Sources: http://www.sciencedaily.com/releases/2007/... & http://www.washingtonpost.com/wp-dyn/conte...