CONUS 36 month requirement?

I've recently entered my window to reenlist but just prior to that I was put on assignment. I have corresponded with branch to see about other assignments but they are telling me that if I reenlist, I must serve my 36 month requirement as my assignment before I can request another PCS. I was understanding that there was typically a 12 month requirement before I can request PCS, a school, or reclass. When did it change to 36 months?