I think that I clean install every 6 months or so... I just format when xp becomes unstable and goes all slow and stuff or when i change something big in the hardware. Running on a 3 weeks old xp now, because i changed my hdd.
I think 6 months is going to be the popular answer...Thats how long it takes for me to make xp unstable (through tweaks and downloads). A year is too long and I have not got Ghost working smoothly yet.