Apple (and most other OS/Software vendors) generally provide support for up to 5 years. Eg, Apple have provided Classic for OS X for 5 years.
I suspect they will continue to provide updates to OS X for PPC for up to 5 years (and probably for most of their other software too.
If they didn't do that, then there would be mutiny when all their customers who bought PPC Macs in 2005 or 2006 would not be able to buy updated software for their near new machines.