Apparently forever. Now many physicists are saying the universe keeps resetting, and there never was a "Big Bang."

Perhaps Les Barker has the answer:

we really don't know. i was listening to a show on the science channel this week on this very topic. some believe the big bang happened from a singularity, but quantum physics, which works for very small things but runs into difficult with larger things, says nope, no such thing as a singularity. others thing we're just one of a series of universes that bang into being, run their courses, shrink back down and bang into the next universe. but this hasn't been figured out yet. they're working on it!

g