I only have a 9GB HD, but I defrag once a week. Sunday is my computer upkeep day when I clean up stuff, clear out old crap, back up files and then do a complete virus scan, complete disk scan and disk clean up and then do a defrag.
The first time always takes the longest, especially if you use the feature where it puts programs most often used near the front of the HD. But it shouldn't take as long the next time if you do it on a regular basis.
If I install or uninstall stuff, I always do an additional scan and defrag, just to keep things running smoothly.
Have you run your SFC yet? I'm sure you'll have more questions when you get around to doing that (System File Checker).
Good luck.