
Up to a week, but a lot depends on what is usual in the breed and the bitch line.
On what days was she mated? Sperm can live for up to a week in the reproductive tract, and at least for four days or so, so your bitch may not have conceived for quite a while after mating.
Last year I ahd a bitch go over by 4 days (usually a couple of days early), but when Vet checked her 4 weeks after mating he was sure fetuses were younger than 4 weeks.
Did you have her palpated or scanned?