How Long Is It Ok For A Dog To Go Between Delivering Puppies?

Any longer than 2 hours would need veterinary advice.
Bitches do sometimes have a break shorter than this between pups, before starting contractions again. However, if she is pushing and straining without a pup emerging for more than 1 hour I would seek advice.
I think your bitch needs an emergency caesarian.

It would be around an hour and a half before you start worrying!