SpectroMagic is 17 minutes from beginning to end.
If you count the opening announcement by Jiminy Cricket through his "goodbye" at the end.
It comes into Main Street between the Fire Station and the Barbershop, circles counter-clockwise around the flagpole "island" and then goes on up Main Street toward the castle/hub.
If Spectro starts at 8pm, and you can find a viewing spot near the train station (as was suggested) you can be out the front gates and gone by about 8:20pm.
BTW, if you exit EXACTLY when you FIRST catch a glimpse of MINNIE MOUSE (she's the last "big star" in the parade) you'll beat a few hundred people to the exit.
Alternately, if you WAIT until her float has PASSED BY, you'll probably be in the THICK of folks exiting.
(I know the timing of Spectro very well.)
LASTLY, I would not wait until just a "half-hour before" to try to find a good spot up near the train station.
It fills pretty fast at the front end of the park because SO MANY folks have the exact same idea as you described.