I always crash into a depression after mania, but it doesn't last very long for me. As Adel said, however, it does depend on the person. You may be lucky and not crash. You may end up with a long depression. It all varies, but I would keep an eye out for your usual depression symptoms to try to curb it off.
not necessarily, i landed smoothly and stayed almost 3 years now without depression. but it depends on the person and the drugs you take to maintain yourself away from depression. most people say they crash, i didn't so i guess you have to wait for other responses.