While it's advisable to take your dog to the vet as soon as they show signs of diarrhea, constipation is a common problem in dogs, and it usually goes away on its own.  Most dogs can pass a solid stool within 8 to 12 hours, but some can take up to a week. In such cases, it's best to consult with your vet on how long you should wait before bringing them in for treatment.