It depends upon your calorie count. Some people consume 400-600 calories daily while hard gainers take more. It is always a good idea to start off with a small calorie level to make your body adjust to its effects.
Tailor the daily consumption of mass gainer around your daily targets and the nutritional value of a mass gainer. Keep a check on the macronutrient of the gainer and the calorie count so you can take a correct dose of a supplement. Take two scoops daily if your goal is to add mass. It will help you supplement your calorie consumption with macronutrient. Do not take more than two scoops else you might face issues.
You can take it anytime: in the morning, pre-workout or post-workout. It will provide you protein and fats to inhibit muscle breakdown and elevate bulk growth. It is recommended to take gainers before or after a workout. They will steer your insulin cycle and inject amino acids into muscles, making you perform better at a session.
You can use any liquid in your shake based on your preference. A large number of people choose milk to incorporate into their shake to gain additional calories; you can do that too. However, if you do not want milk, you can opt for water instead.
Creatine is a one-off ingredient when used with mass gainers. Consume one scoop of mass gainer with creatine pre-workout and whey protein after a workout to speed up your performance at workout sessions.