i dont find it worth my time to use the default windows defrag. I use system mechanic pro and use their optimized defrag option, and there is a considerable difference between that and the standard defrag. Its taken me up to 2 days to defrag, but its worth it.
Its the one function I always forget about, try to do it once a month, I do do system maintenance each weak but I'm a download junkie though, man do I hate them viruses.
It really kinda depends on how much you're fragging up your HD. If you're installing and uninstalling alot of programs or creating/deleting a lot of files then you should do it often.
If you've changed very little on your system in the past month, then it's hardly necessary.