I defrag the server's drives about every month or so. The length of time a defrag takes really depends on how much space on your drives you're using. My 30GB usually takes about 2 hours, and the 80GB usually just runs through the night.
That was when the computer was being an utter bastard, though. Nere's suggestion of once a month is pretty good; my dad tends to do it once every three. I think how frequently you defrag affects the length of time it takes as well as the size of the drive; I could be wrong, though. If you haven't defragged for eons, for example, it could tak a while.
Dragomorph
"Actually, what I know about a computer's internal works would fit on the head of a pin, but whatever."