Most cases of Giardia in dogs will go away on it's own without medical treatment.
But if the Giardia in your dog is more severe then the dog will need medical treatment to get the Giardia to go away.
If the Giardia in your dog is not as serious then it will usually go away within 7 days but if it lasts longer than 7 days then you may want to take the dog to the vet to get treatment.
In more severe cases of Giardia it can take a few months for the Giardia in your dog to go away fully even with vet treatment.
The reason it can take so long for the Giardia to go away is that the intestines need time to repair themselves after the infection.
If your dog is suffering from the infection Giardia and has diarrhea or chronic diarrhea then medical attention for your dog is needed.