The average length of a dog pregnancy is approximately 63 days, or two months. The due date can be about one week on either side of the predicted length. There are also other things that factor into a pregnancy length.
Like in humans, there are factors of a pregnancy that can differ the length. If a litter is larger than a typical one, then the length of the pregnancy will likely be lessened to a shorter duration than 63 days. However, if the litter is smaller than normal, then the pregnancy length will tend to be longer than the planned 63 days.