Weeeeel, osx was on an 18 month cycle of releases up until about 10.5ish. Then they toned it down, mainly I think due to there not being much to do oswise. (COUGH, memory dedup in the vm subsystem COUGH).

I think of any software an OS would benefit the most from a timed release schedule. Ref: Linux/*BSD, don't ref: Hurd.