[RedsBaron]

Pitcher abuse is primarily a factor of pitches thrown in a game. Studies have shown that the potential risk of serious damage to a pitcher's arm greatly increases after 120 pitches in a game. A pitcher may rack up 200-240 innings a season without undue risk if his pitch count per game is held to the range of 100-120. His risk of injury can actually be greater if he pitches fewer innings but in the games he does pitch he has a pitch count of 125-140.
There is a good discussion of this concept at pages 866-867 in "The New Bill James Historical Baseball Abstract" (it is in a brief article discussing Don Sutton and the advent of the five man rotation). Keith Woolner also wrote a good article about this issue, entitled "Five Starters Or Four," which is found at pages 74-94 in "Baseball Between The Numbers."
This isn't to say that some pitchers cannot pitch many more innings, with much higher pitch counts, without evident ill effects. They are extremely rare though under present playing conditions.