Fenbendazol is used for large animals, dogs and people. For dogs, three days is sufficient and my experience is that repeat after 2 weeks is superfluous. You don't expect to completely eradicate giardia forever. Only need to give your immune system the upper hand. Eliminate the conditions that allowed the eruption.The standard protocol in Brazil is 5-7 days of albendazole 400mg. Then repeat 15 days after.
However, using the 10% large animal (cow, horse) liquid veterinary product to save money and convenience, the dose per kg dog is ten times the large animal label dose.