How long can a dog be in labor?

I have a dachshund, she is about four years old, and has never had puppies before that we know. We got her given to us, and lo and behold she was pregnant. I know nothing about dog labor, but she has been panting, and crying since about 9 PM, and it is now 8AM and still no puppies. I have not seen any actual pushing yet, but She is having contractions, her belly gets hard, then relaxes, so I figure these are contractions. How long can this last, and should I call the vet yet, she has not been pushing or anything yet?