I've been accepted to the polisci PhD program at Brandeis, and it's currently one of my two top choices. This morning I had a good conversation with a prof (who, coincidentally, mentioned that he does a lot of work with the sociology department), and have had encouraging contact with the department in general. I too was wondering about the length of time grad students there typically take to finish (one of the rankings websites listed the percentage of their students graduating within five years as something like 12%).
Department and university culture surely plays a big role in this, but it seems to me that the student has final responsibility for how long it takes him or her to finish. Of course, if they keep offering to extend the funding package, anyone might be tempted to stay around, no matter how responsible they are.
Good luck with your application!