I don't really know what to vote for. Maybe you should've made less possibilities and bigger differences (i.e. 5 min, 10 min, 15 min, 20 min, 25 min). I don't think we really get a big use out of this thread, maybe a tendency.
Also it's hard to define epic for me, but maybe i would consider the following the best prog songs ever (prog songs!!! there are other bands i like equally, but their songs are not as long, so i wouldnt call them epics):
Anglagard - Jordrok (11 min), Caravan - Nine Feet Underground (23 min??), every GY!BE song (~20min), Pink Floyd - Atom Heart Mother Suite (24min), Camel - both Nimrodel (9min) and lady fantasy (11min), genesis - both musical box (11min) and cinema show (12min??), King Crimson - both Starless (12 min) and 21st century schizoid man (7:30min)
Maybe you can't call 21st cntury schizoid man an epic, so I won't include it in my vote... so the shortest one is: Nimrodel (9min), i voted for 9 min!