Well considering the original DS came out in 2004 and the Wii and PS3 didn't come out until 2 years later, I think it'll be a similar time frame for the 8th generation (yeah the 360 came out in 2005 but that was largely because MS was rushing it out because the original Xbox did so poorly.)
So I think a reasonable time to expect the next CONSOLE generation to start, assuming 3DS comes out in 2011, would be 2013. However, I could see MS being the first ones out the gate again like this gen, maybe pushing their console for 2012 if Natal has little or no impact on 360 sales.