(to continue to derail this even further...) I bet it's infinite (in time and space), i.e. Big Bang wasn't "the beginning" of everything. If anything (pun intended
), it was a beginning of a cyclical expansion of the
part of the universe that we happen to be capable of observing. When I was in my teens, a friend and I had visualized this model of infinite collection of "bubbles" (each bubble being what we deem "observable universe"), pulsing between the BB state and the state of having expanded to the point where various forces would start pulling it all back together, until it reached the BB-level super-dense "dot". Back and forth, forever.
Another idea would be that it all will just continue to "expand" (as we're currently interpreting our puny measurements), as it always has been, forever, with "new" matter being continuously "created" from sub-levels so "thin" that we can't possibly register them (the "quantum field" and then continually "zooming in" further and further, again to infinity).
All speculation of course, but just seems to make a hell of a lot more (common) sense than the "nothing existed before the BB, and then at some set point in time everything started". To me anyway.
Smells too much like the first few verses from Genesis, with some super-orthodox folks still denying validity of methods like carbon dating and believing that the universe is less than 6000 years old. Same idea, just different scale. IMO there is no "time limit" to it.