interesting forum thread on treating dogs with fenbendazole. People are suggesting treating for 7 days then repeating the same dose after 15 days. One person did a 3 day course every other month to keep giardia from coming back. http://www.champdogsforum.co.uk/board/topic/129558.html
It may be interesting to repeat Fenbendazole for 3 days after 15 days from the first two 7 day treatments.