Anywhere from 3 to 6 tbs would work depending on your taste. Use an eyedropper or pippet in a twelve ounce sample and find out your taste. Assuming a drop is one mL than use this conversion(I have done the math)
1.5mL in 12 oz. = 3 tablespoons in five gallons
3 mL in 12 oz = 6 tablespoons in five gallons
Or you can keep the extract right at your bar and have your choice of a plain stout or a vanilla one.