How long will it take me to gain weight?

It's rather impossible for anyone to guarantee you will obtain a certain amount of results in a certain amount of time. Everyone is really different, and we don't all achieve results at the same rate. Your progress is going to depend on a lot of factors, including your entire diet, exercise, genetics, etc. 

If you are looking for a good rate of weight gain to shoot for though, I would recommend about 1-2 pounds per month. This should equate to just a couple hundred extra calories added into your diet (on top of what you need to maintain your weight). Remember while monitoring your weight, you can always modify your portion sizes and meal frequency accordingly. Your Gainful protein powder does contain calories and protein to it to help your diet, but there is no magic supplement out there. Simply drinking it without regards to anything else doesn't automatically guarantee results. Remember that the rest of your diet and food intake is very important as well.