Hi there. Firstly congrats on the pups! The situation doesn't sound like something I would be concerned about at the moment. Dogs can take many hours to give birth and it sounds like she is doing well. Causes for concern are:
1. Persistant straining with no pups being passed. There is no strict time period for this but I would say straining with no success for 1-2 hrs is concerning.
2. Passage of greenish/brown fluid
3. If spaz becomes obviously distressed for prolonged periods of time.
If any of these things occur, she may have what is called a dystocia (problems giving birth) and may need veterinary assistance such as hormones or a caesarian.
Hope it continues to go well and good luck!