It depends on your definition of pushing really

If she's labouring (lying down panting, back leg coming up to the belly and changing position) it can be a few hours, if she's lying and actually straining (and you see the vulva bulging as she does) it shouldn't be much longer than an hour, sometimes less.
Hopefully as I type you'll be seeing piglets born, but if not you might need to think about doing an internal just to check none are stuck (remembering to give an anti-biotic jab if you do)
Good luck & HTH
Karen
