Minimum 2 years! I mean like bare minimum.
The earliest they can announce a console is 2010 E3...and I am 99% sure they wont as they have so much going for them on the 360. If natal is a huge falure they may panic and announce it for 2011 Q4.
However, Im guessing 2012 Q4 for a new console. It should be the logical guess for most people here.