Well, i guess you should do it everytime you install or uninstall something. Defrag is very important for have an optimized Hard Drive. I do that almost every weekend.

About once a week I go into Disk Defragmenter and click "Analyze". I only defragment if Disk Defragmenter tells me I should after it has analyzed the drive.

I wondered this as well and asked my co-workers who are a large IT organization. No one really has an answer but since I spent so much money on a high performance system, I wanted to get the best from it so I purchased Perfect Disk. It defrags on a schedule which I set at every other day. This is likely more than needed but I when I was using Perfect Disk manually I noticed that, the more often I defraged the more less time it took to complete the defrag.