One never knows, but I'd personally be surprised if they pulled that trigger too quickly. I'd personally put my guess around the beginning of 2011, which would make the last PPC macs about four to five years old...
OTOH, I would guess that the number of users who use the latest iTunes features or the latest iPods on PPCs is lower -- I don't know numbers, but I'm guessing the number of people syncing iPhone 3GS's to G4 Macs is few and far between, and likewise that few customers are renting video content from Apple. So it might be that if they stop supporting new iTunes versions for PPC, customers on PPCs wouldn't miss much.