For me, it started in my left knee and then spread to my right as well after arthroscopic surgery on the left knee. It started gradually, where I thought the pain in my right knee was just from overcompensation after the surgery for awhile, but I soon came to realize after the pain kept getting worse that there was something wrong. At the time I didn't have the diagnosis of RSD so I didn't understand that my pain could spread, but eventually and gradually (over the course of maybe 4 or so months) the pain and symptoms in the right caught up to the those in the left.

Now I'm experiencing just the burning pain up and down both of my whole legs. I think that eventually I will probably have all of the RSD symptoms throughout my legs, and the spreading is happening a bit differently this time.

So I agree with what others are saying, that it happens differently for everyone and can even happen differently for different cases to the same person. Those are my experiences, though, and I hope it can be of some help to answering your question!