Not that long, lah! 
It would partially depend on how fragmented the data on your HD is,
and how much free space exists on the HD for the defrag utility to
use as a work area.
My primary desktop PC has a 360 GB HD that's about 80% full. I do a
full defrag on it every two weeks or so, and the process usually
takes about 4 to 5 hours. I'll start the defrag before going to
bed, and it'll be done by the next morning.
