for "innings pitched".
The career record for innings pitched is 7354 2/3, held by Cy Young, and will likely never be broken. Second place all-time (Pud Galvin) had less than 6000. By comparison, the active leader (Roger Clemens) had less than 4000, entering the 2001 season. Pitchers don't pitch as many innings as in the past for a variety of reasons (rule changes, rise of relief pitchers, and changes in pitching techniques, among others).
Will White of the Cincinnati Redlegs holds the single-season record with 680(!!) IP in 1879. By comparison, Jon Lieber led the majors in 2000, with a "mere" 251 IP.