Average 4 years.
New Versions of OS X systems tend to work for computers up to 4-5 years old. which is the average useful life span for these computers if you are keeping up to date with software and newer technologies. So about 5 Years ago we were 1 GB of RAM is like 4 GB of RAM Today. 40GB drive is like the 250GB Drives. 1 Power PC 667 Mhz system is like a 2.33Ghz Core 2 Duo. Wireless back then was a gee-wiz nice to have feature, today it is standard. 32 Megs of Video RAM was considered OK, Now it is 256 Megs. No USB 2 (Only USB 1), DVI and LCD Displays were just starting to get in the sub 1k range for 14" displays. Just being able to Read DVDs was a big thing, Now we have burners for DVD standard and HD-DVD/Blueray is the big thing. OS X applications were just starting to get the majority over classic applications. USB Thumb Drives were Really cool and expensive paying $50 for 128 MB. Most people still used Dial Up to connect to the internet. JavaScript in websites were used sparingly. And Strongbad still had an Accent. Things have changes a lot in 5 years. So that is a rather fair range to keep support for you OS.