Martin Middlebrook gives a good breakdown of Bomber Command tour lengths in "The Nuremburg Raid". IIRC, Miles Tripp in "The Eighth Passenger" also mentions a period when Bomber Command tour lengths were set at 35 operations. Raids after D Day in Occupied Europe originally counted as 1/2 an op unless they crossed a certain longitudinal line, although I believe that later changed. Of course, in any case an operation had to meet certain requirements in order to "count". Interesting point about the losses among crews starting out on a second tour, too. We could have a great discussion about that.
An 8th Air Force tour was 25 missions.
Hope this helps a bit.