The number of weeks in a rotation is variable depending on our church's schedule. Sometimes we have a special Sunday with no classes, and have to plan 3 week rotations.
Other times we have a story and great workshop ideas and want to do five weeks. (In rotation, how many classes you have is kind of irrelevant to how many weeks you spend on a story.)
We typically rotate four workshops a month, September through May - on those Sundays when there is a fifth month - we simply start the new unit on the fifth Sunday.
Odd thing is, those "fifth Sundays" are often in months with Easter and the Christmas pageant rehearsal and usually takes care of that odd Sunday.
Since we stop our workshops in May - the first Sunday in June we have a review and a generally good time- we also use this time (church school hour) to have a brunch for our teachers. Our school systems get out the second week in June so our church school classes are over at the end of May. This has solved our problem with the fifth Sunday.