2.6.<odd>: still a stable kernel, but accept bigger changes leading up to it (timeframe: a month or two).

2.<odd>.x: aim for big changes that may destabilize the kernel for several releases (timeframe: a year or two)

<odd>.x.x: Linus went crazy, broke absolutely everything, and rewrote the kernel to be a microkernel using a special message-passing version of Visual Basic. (timeframe: "we expect that he will be released from the mental institution in a decade or two").