Never.
I've carried the same install of Windows XP through 4 years without having to reformat or reinstall. During that time, I've upgraded the CPU, Motherboard, Video Card, Sound Card, TV Tuners, and even hard-drives. The Motherboard / Video Card upgrade was multiple times and even as drastic as going from AMD Barton Socket-A to AMD X2 Socket 939 and then to Intel P965 LGA775 and from ATI 8500 to 9800 to X1800 to X1900 and then to Nvidia 8800 GTX. Even when I upgraded hard-drives I imaged the old drive onto the new using Ghost / Tru Image or equiv software.
The only time I reformatted was to change OS from XP to Vista or when on my second system I was truly experimenting with various install options like nLite or the various versions of Vista (Business, Home premium, 32bit, 64bit).