As a general recommendation, we suggest drinking 1 serving per day. A large part of it will depend on the rest of your diet, and how much protein you are consuming through your food intake. The idea is to use the protein powder to help meet your protein needs for the day, along with the rest of your diet. However, you don’t have to be super meticulous and count all of the grams of protein you eat. If you are consuming quality protein foods throughout your day at your meals and snacks, 1 serving a day should be sufficient for you.
Some people who have a more difficult time including protein into their diet (such as those following vegan/vegetarian diets) may benefit from an extra serving or so. If this applies to you then I would recommend spacing the servings out throughout the day. Taking two servings of your Gainful blend at once will be a lot of protein, and your body can only utilize so much as once.
If your metabolism is very fast and you are having a hard time gaining weight, remember that protein isn't the only important thing. You could eat as many grams of protein as you want, but if you aren't consuming enough calories then you won't gain weight. If this applies to you then instead of adding an extra scoop of protein, I would try something like making a smoothie with it. Adding things like milk/soy milk, fruit, peanut butter, oats, etc. will greatly increase the number of calories and other nutrients in there (including some extra protein!). This should benefit you more if you are struggling to gain any weight than simply consuming an extra scoop of protein powder.