Oftentimes the end of the manic high will result in a drop into a severe depression. This was only the case for one of three episodes I had though, although I was hospitalized for the last two (and as a result medicated on the way out of the manic episode, which might have prevented the depression.)

Regarding how long will it last: blogs.psychcentral.com/bipolar/2008/10/how-long-can-a-manic-episode-last/. My first one only lasted one week I think.