I wouldn't be at all surprised if Capaldi was a one-season Doctor, TBH
Depends on whether or not there's a gap year in 2015 as has been rumored.
I assume that the eighth series is Moffat's last. If there's a gap year to get a new producer up to speed with the program (as was done in 2009), then I could see Capaldi departing with Moffat when the series goes on hiatus. He's a working actor, after all. If the series doesn't go on hiatus when Moffat leaves, I could see Capaldi staying for three or four years.