depending on whats running in the background that could be right
from what i recall my p4 3.0 with hyperthreading (w 1gb of ram) takes about 20-40% longer than the movie to encode with NOTHING else eating up the cpu.
If you have some stuff running in the background or are encoding off a slow drive (maybe external or the likes) it could easily be extended to that long.
Try turning off your virus scanner and if the avi file isn't on the hard drive already put it there. Or check if you have multiple hard drives set the output to a different drive.
also try installing the latest codecs
Oh and also if you areusing ANY of the image filters it WILL be that long. If i try to up brightness by even 10% it extends a normal encoding session up to 3x what it would normally take