Liquid diets have proven to be effective fr a period of time as long as you are getting all of your nutrients in your smoothies. How much weight one can lose on smoothies alone in a month depends on what other steps you take to help lose the weight. Smoothies are high in sugar and you are going to have to exercise. Every body reacts differently to a revised eating plan. It's not really healthy to lose more than 10 pounds in a month.