I have raised many litters. I agree with everything LuckyRVT said. It is normal for her to take all day to deliver a big litter. Probably about halfway through she will take a 4-5 hour rest. It is normal for it to take up to 2 hours in between puppies. If it takes longer, don't panic. As long as she's relaxed and not in pain, she's ok. However, if she is in pain or pushing for more than an hour without producing a puppy, that could signal trouble. Make sure you count the placentas as they are delivered. They may or may not come with the puppy. Make sure at the end of the whelping that you have counted a placenta for each puppy. If she retains one, she can get a serious uterine infection that could lead to death.
Lots of people say to not interfere with whelping. I, for one, assist every time. I raise English mastiffs and they are notorious for having trouble. I have one female that I have to help pull her puppies out every time. If you have to help pull one, make sure you pull down between the mother's legs toward her head.
I hope this helps, if you have any questions or trouble, let us know!