Four and half years from now, in time for Christmas is about right and seeing as the 360 had a headstart on Wii. I could see Nintendo's next and a new Xbox released around the same time.
I don't believe all this ten year life of PS3. Technology moves on Sony would get a new machine out there 12-16 months after the competition.
If like people its going to finish up being a three way split market its hard to see what will happen. PS2 is going strong so long after because it was so successful. If the spoils are shared then maybe more motivation to kick start the new wave.