When do you think it's time?
He seems to be getting a little but slower, both in reactions and scoring, and I think it's time he retires to focus on Test cricket. If he retired now, he gives us time to work on a replacement before the 2011 World Cup.
However, the 2011 World Cup is something he will want to play, and probably should play in. Australia will look up to him, and when he's in form I'd like nothing more than to see him play.