Foreign culture and no. of soldiers you have in the city, as far as I know. Each turn that you don't have the required number of units to put down any chance of revolt, a hundred sided pseudo-random die is rolled. I believe you can see this chance by hovering over the culture bar in the city screen, but this may just be a BUG feature.
Edit: I misunderstood the question as revolts are always 10 turns. If they keep persisting (this is what you likely meant) look towards the above.