How much disk space should you keep free?

How much free disk space (Local/OS disk) should you have in order to not slow down your computer?

for what OS?

For windows 7 at least 5gb for a 64bit installation

I think for defragging you need 15% free space for it to work properly.

yeah windows 7 64 bit. Thanks a lot guys ill be safe and keep 15% fre

I'm not sure who said this but i was always told to keep at least 10GB free on the OS drive, but dont quote me on it...