How long does it usually take someone to get his PhD in computer science from MIT or Stanford? Does it differ if he has master's degree from another university?

Typically, its about three years. Formerly, it was difficult to get a PhD from such schools without doing some real research for a dissertation, and people sometimes took quite a while to produce such a result. In the last ten or 15 years, however, I have seen a few accepted dissertations at these schools which are so banal that they once wouldn't have been considered adequate for an undergraduate term paper. When I asked a few professors about why such work had been accepted, the answer was that the candidate had done a lot of work, even if it wasn't worthwhile.