Apple cycle time

How often does Mac upgrade its products? For example, before the iMac was upgraded last October, when was the previous upgrade? I will be buying a Mac in the future, but maybe not for a year or so. I'd hate to plunk down big bucks on a computer that is near the end of its lifecycle. Does anyone have any insight?

It's usually every ~250 days,
This site is extremely helpful in these mattersMac Buyer's Guide: Know When to Buy Your Mac, iPod or iPhone

Thank you, McYukon. Very informative and just what I was looking for. It looks like the iMac may be updated again within the next two years, for sure.

Belay that. It looks like the previous iMac upgrade was in Oct. 2005, so it took them four years to update. I guess it is a crapshoot.