Whey should be very fresh. It will still be good if you have less than a gallon. Place whey and whole milk in a large stainless or enamel pot. Heat to 195 [degrees] F, remove from heat and add lemon juice.
Stir the mixture for several minutes, watching as the juice produces tiny curds in the liquid. Pour the hot whey and curds into a cheesecloth-lined colander and allow to drain and cool for 25 minutes. Remove curd from the cloth and break into tiny pieces in a bowl. Add salt to taste. Add a little whole milk or cream for a richer ricotta. Will keep for one week in the refrigerator.