I read an interview with Patrick Dempsey in which he said he hadn't known at all since his contract ran until 2016.
He said that he found out in March. He will miss the other actors but he said that the job was 10 months/yr and
15 hrs per day. He believes that these hours took a toll on his marriage, and were partly the cause of his divorce.
He is apparently an avid race car driver and looks forward to having much more time with his children and with racing.