How long is the average life of a Mac?

I've had my MacBook for three years, and for the most part it seems to be running great still after all this time (battery life aside).  I'm not looking for a new mac right now I just wanted to get an idea about how long people usually use their macs before investing in a new one.  iMac,  Macbook, Mac Mini, Mac Pro, whatever.  Also tell me what model it was and what version it was running when you upgraded.