I've seen it suggested at 1/8 of a teaspoon per every 10 pounds of the dog. I also saw 1/4 teaspoon per every 1 cup of food. With any supplement, I suggest starting slow and building up to the full dosage.
They say a daily amount of 1/4 teaspoon per pound of dog food is safe. Start slow at first to get the tummy accustom to it. Here is an excellent article from Dogs Naturally Magazine.