A dog's pregnancy will last about 60 or so days. They can have contractions for a few hours up to around 18 or so hours. Once they pass into the second stage of labor, they should deliver puppies fairly quickly, but sometimes it can take a few hours between some puppies. If your dog is straining for 45 minutes or more to produce a puppy, you should seek veterinary assistance. Also, if four or more hours have passed between puppies, you should take the dog to a vet because this could indicate a problem. For more information, visit this site - http://www.2ndchance.info/caninelaborstages.htm
Dog pregnancy usually lasts for about 2 months, ranging from 53 to 69 days. However, smaller dogs will give birth earlier than larger breeds, so remember that. There are usually an average of 6 to 10 puppies in each litter. Make sure to bring a pregnant dog to the hospital constantly so that it can maintain the health needed to give birth.