Ask Max: How long does Apple support older versions of its operating systems?
If I buy a used PowerPC computer that runs 10.5, how many years can I expect Apple to continue OS support for 10.5 Leopard?
Well Apple makes no promises about support ranges, but if you look at the support of the previous versions of the Mac OS, you should have an idea. Mac OS X 10.4 Tiger was released April 29, 2005. That was over four years ago and Apple still offers support for Tiger, in the way of security updates and some software compatibility.
If you are concerned about third party software support on your Mac, it may be a good idea to get a used Intel-based Mac. Just because an OS supports a processor type, doesn't mean all software created for that OS will also support that processor.
In general you should have a few years more of support from Apple on that OS, and it will probably be a little longer, due to Leopard being the last OS that will run on PowerPC Macs.