Both major parties are imploding. The Republicans are falling apart as the machine that put Bush in power has reached the end of the road and the price on their various illegal schemes is coming due. And the Democrats can't get organized enough to vote for a bill that says "Democrats are cool." If this situation continues into 2008, the country will be primed for an outsider to come in.
Perot managed 18% of the popular vote, and that despite some serious problems with his campaign. Bloomberg is a much more experienced politician and isn't likely to such blunt mistakes and should be able to organize better ahead of time. I do think that if he really wants to win he needs to move soon.
If his goal is just to shake up current politics and force some issues to the front, he can wait longer. He has the money to make a splash even if he waits till the last minute. Remember that Perot may not have won, but he did force deficit spending by the federal government to the level of a national issue and force discussion of other things the major parties would have rather left unsaid.