How often should you defragment your hard drive?

It would depend on how badly your drive gets fragmented. If it does so on a daily basis, you could simply install a tool that will analyze and run in the background without manual intervention. I run mine every week after a disk cleanup. I also agree that atleast 20% free space has to be maintained. Check the level of fragmentation first, if it is over 5% then defrag the drive, also to be considered is the free space , always leave at least 20% free, the hard drives today do not cost much so there is no need to be skimpy here. Just as a side note, If you are installing and uninstalling programs/software often then running a disk defrag after every install or uninstall will keep your disk healthy and keep the time it takes to complete a defrag to a minimum.