If you want long-term studies, look at Jamaicans who smoke all their lives. In the 70's, this was a primary source of information for surveyors who wished to look into the effects of cannabis.
As previously mentioned, ANY smoke is harmful, but there are such things as rates of damage, recovery, and basically how much of an effect a specific test item has on your lungs. An example of this was done in a test between clove cigarettes and regular cigarettes, where suprisingly the damage was not only less from the clove cigarettes in comparison, but the carboxyhemoglobin levels fell faster as well. I'm surprised this method hasn't been adapted into an in-depth study on cannabis.
To be blunt, cannabis poses no long-term lung damage/cancer risks. In fact, THC is known to protect against cancer.