There are some pretty standard guidelines with birthing puppies that we consider to be emergencies. These are:
1. 45-60 minutes of strong contractions or pushing and no puppy comes out
2. If a puppy is stuck or the mother is very painful.
3. If more than 4 hours passes between one puppy and the next.
If any of these occur, it is best to get her in quickly for both the health of the mother and the remaining puppies.
I hope that helps! Please let me know if you need anything else!