According to my ortho, what Leebie said!
Orthodontics work by putting force on the teeth, which dissovles the bone and then the bone reforms "on the other side" stabilizing the teeth in their new position. So I think for most of us (especially without severe bite issues), things happen REALLY fast and then it's a waiting game.
We've closed my diastema, closed the secondary spaces, opened my bite, now it's time to tweak and then stabilize. I am still hoping for 18 months total. Fingers crossed!