The justices have a fairly stiff schedule during the season; they are expected to do all of what the job entails, even though they're assisted by clerks. Historically, when a justice is no longer able to do the work, he contacts the President and lets him know that a replacement will be needed; that has generally worked. (Of course, in the current Court, it will be hard to know when Justice Thomas is unable to do the work, since he's doing so little of it now. He is known for never asking questions and writing as little as possible.)