That sounds a little long
I had a 20 minute quicktime movie (about 2.8 GB)
I exported it from QT 4.1 using Toast 5's mpeg encoder
on my Pismo G3 400 MHz it took almost exactly 5 hours
on my Ti G4 400 MHz it took almost exactly 2.5 hours
I have to assume that iMovie's DV streams, or whatever it uses, are less compressed than my quicktime movie source file, and therefore they would be easier to compress (into a different codec--mpeg1), so your time should be less than 5.6 hours, but there are a lot of assumptions there....