Generally the amniotic sac will break or emerge a few hours before birth. Though there is no set time limit. The key is to watch for the stages of labor to determine how close she is. It sounds like she is in stage 3 which is the final stage. You should notice contractions, panting, and nesting activity by now as well as maybe some light or heavy pushing.
Not every dog will follow the above guidelines, but they are the most common. You will want to be prepared by having a few things on hand.
You will want to be there when she gives birth to make sure there are no complications.
It can pull back in some if it is just slightly out, though it is likely that the sac simply broke and she cleaned the mess up. Keep in mind the female will attempt to clean every trace up of the birth from instinct.
Contractions can start before or after the water sac breaks. If she is in hard labor for more then 4 hours with no puppies then she will need to see her vet. Hard labor is:
If you are seeing an amniotic sac and no puppy arrives within 20 minutes you should seek vet care. The individual sacs do not normally break before pups are born but come off during the birthing process or when mom opens them up after birth.
If the sac protrudes and goes back up inside then something is going wrong with the birthing process. It may be the pup is too large to deliver, it may be two pups are trying to come at once, it may be the mother dog has not enough muscle or energy to deliver the pup but in any of those situations seeing your vet, getting an x-ray to see what is going on or other help may be critical to your dog and the litter surviving.
I would not wait even one hour once hard labor has begun and the sac protrudes the first time.
If no puppy has arrived at this point you should call your vet and make sure you can get vet care to help with this delivery. Waiting may risk the life of mom and pups.
Good luck with the litter!