ROWA suggest around 100g per 400L on a saltwater aquarium and 100g per 800L on a freshwater aquarium.

If the aquarium is heavily stocked then this ammount can be increased by 30% but it is best to use small amounts that is changed more frequently than large amounts changed less frequently.

Please see the link below for more information.

http://www.theaquariumsolution.com/rowaphos-capacity-information