At a joint press conference with Canadian PM. See here. So far Obama has reiterated what he has already said: transition must begin now, but the U.S. will not determine what that transition will look like. Obama seemed to signal that Mubarak sticking around till September might be a tolerable option, but again, wasn’t specific enough for us to pin down:
“Once the president announced that he won’t run again, and since his term is up relatively shortly, the key question he should be asking himself, is how do I leave a legacy behind that will help Egypt through this transition – and I hope he will make the right decision.”