Any time I think his advice would be particularly useful in a situation. If I ask for some time with him I'll give him an indication of how urgently I feel I need to see him so he can fit it into his schedule accordingly. He's pretty good at finding time at short notice for people who need something in a hurry, but if I'm just wanting to talk over something with no great urgency I'll let him know so he doesn't feel any need to squeeze me in (potentially sidelining someone with more pressing needs who didn't want to make a fuss) for something that can wait.