Rebuilding a PC: Why?

How often should you rebuild your PC? What kicks it off for you?

Is it proactive? For example:

• Length of time since last key event
• Amount of data
• Previously determined OS attempt

Or is it reactive?

• Due to performance?
• Perceived slowness?
• Hardware failure?

The key is to know why you're doing it. Not all reactivity is bad (hardware failure, for example, you can't control or really prevent).

However, what if it's a server? How often do you rebuild things, even just the OS? Yes, I know up-time is king. I do.

However, if you can unlock more efficiencies through an OS upgrade, why don't you? Why not be proactive?