We went to 2 parade in Sept.
Spectro was about 20 minutes.
The daily MK parade (I can't remember the name) was 20 minutes the first day, and only 12 the 2nd. I don't know why they didn't show all the floats on day 2. I talked to several people around me, and they were puzzled as well.
Even though the parades were fairly short, it also depends on where in the route you are standing and how long it take the crowd to disapate.
Sorry, I don't know about the MGM parade.