http://zap2it.com/2015/12/peter-capaldi-may-leave-doctor-who-season-10/

The actor tells The Telegraph (http://www.telegraph.co.uk/culture/tvandradio/doctor-who/12056889/Inside-the-Tardis-behind-the-scenes-of-the-Doctor-Who-Christmas-special.html): “I love Doctor Who but it can be quite an insular world and I do want to do other things.”

they should have enough time (sometimes more than six months) between series for him to "do other things"

I don't understand why these actors can't commit to 5 years on a show that is going to increase their exposure and popularity exponentially. They don't even have to do that many episodes per series, and a couple of specials every few years. I think it disrespects the institution when they pull these stunts.