I have had bitches with a little discharge every now and again (dark brown, no smell at all), until 5-6 weeks after the litter was born. Just monitor the color and smell - anything "fishy" or smelly, or looking like tomato soup - get her to the vet ASAP!
5-6 months caused by subinvolution of placental sites. We have had this condition twice over the years, after consulting with our repro vet, as long as the bleeding was slight, not to worry as it will resolve itself. The above condition discharge shows as bright red and should be very slight.
On ANY abnormal discharge, if you are not sure, please consult with your reproductive vet. We are quick to consult our vet on any abnormal discharge both during pregnancy and after whelping, I urge everyone to do the same.