I've never gone more than overnight with leaving a cake in the fermenter...and that's with leaving a layer of beer on it, keeping it sealed, as cool as possible AND washing it. But it's dicey to do it..Personally you should do your racking off the yeastcake while your wort is chilling in the brewpot, so there's no gap, and risk for something to infect the yeast.
If you are not going to be able to pitch on the cake right away, then I think it's best to wash and jar up the yeast...or at least pour it into sanitized mason jars (beer trub yeast and all) and then wash it later.