How long after your waters break do contractions normally start (naturally, not induced).

My waters broke at 1.15am this morning, and so far, I don't think I've had any contractions. Nothing different from the Braxton Hicks I've been having for the last couple of weeks, anyway.

And what can I do to encourage contractions to start without being induced?

ETA: Thought I'd better add that I have already been to the hospital, and am going back later tonight for antibiotics if labour hasn't started yet.