Presidential campaigns have gotten longer and longer, over the last few decades. The 2012 campaign will probably begin earlier than the 2008 campaign began.
My guess is that most of the serious contenders will make it known that their hat's in the ring by next summer.
I'm sure that Sarah will let us know something around the same time as the others, although she will probably time her announcement for maximum effect.
When it comes, look out. All hell's gonna break loose on both sides.
Next summer or earlier. The first Republican Debate last time was May 3, 2007. 90 minutes long, was on MSNBC. Here was who was in the debate: Brownback, Gilmore, Giuliani, Huckabee, Hunter, McCain, Paul, Romney, Tancredo, T. Thompson. 10 candidates by May 3. Fred Thompson and Alan Keyes announced later.
Most candidates, if not all, will wait until after the November elections. Between November and the spring most candidates will announce.