I wait a minimum of 2 weeks before I move to secondary. Secondary is not a fermentation phase (unless you are adding a sugar, fruit, etc). If you add fruit or more sugars (honey for example), you should leave it in that fermenter for 2 weeks. Dry hopping is different, since you are not adding sugar there is no fermentation occurring, and you can rack it onto your hops for as little as 3 days (most suggestions are not to let it sit on hops for more than 7 days). I rack all of my beers into secondary, no matter what I'm doing with them (I want my beers as clear as possible). If you are adding fruit or sugar for secondary fermentation, I might say to rack before too much of the yeast has dropped out of solution. That way you are ensuring that you have yeast to eat the new sugars.
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