You can stay on as long as you want. Whether you should or not is a different matter.
Basically trying to establish a time period that you are going to cycle before starting isn't a good idea. Because once you reach the 10 or 12 or whatever week mark you have decided upon, if you are still making gains you will only be robbing yourself by ending the cycle at that point.
On the other hand, if you are still cycling and for the last 4 weeks you haven't seen any gains, it is time to take a break.