How often does Google Earth update images?

I'm think sure it's random. The reason is because I google-earthed (is that a word?) my parent's store and a few months later (2-3), the image had changed. But when I viewed my old work, I saw my car in the employee parking lot there, and I haven't worked there in two years!

My best guess is that for the busier areas, they probably update the images (via the van that actually cruises down the street taking the pictures) every few months. But in some more rural areas, they take their sweet time.