According to this document:
http://www.afhra.af.mil/shared/media...080424-048.pdf
From spring 1944, the official policy of the USAAF was that there will be no more fixed tour, but that crews will be sent home when they will be "war weary". Application of this rule depended from the theater and the unit. The idea was to have as much crew as possible to support (among other) the invasion of Europe.
In September 1944, with the casualty rate falling in the last months, many air forces had far enough crews, sometimes more than their authorized complement, and the rule changed: "when the theater commander determined that a crewman had completed a combat tour and a replacement was available in the theater, the man should be sent back to the states."
The same document says: "Between November 1944 and May 1945 bomber crews of 9th Air Force averaged 65 missions."