The size of the hard disk matters. In order, 1.2 GB drives defrag faster than 20, 40 and 120 GB drives in that order.
So it can take a very long time depending on the drive size and whether you have a Pentium 100 or P4 3GHz.
I started defragmenting my HD with System Mechanic Pro from Saturday 26th 2005 until Monday 28th and it still didn't finish!! Is that normal ?!?!?!?