Muscle gains are going to be highly variable from person to person, depending on their starting point, personal genetics and training history. Someone that has never trained at all is typically going to gain muscle slower than someone that used to be an athlete and is merely out of shape.
Also, it's hard to define what you mean by mass. Do you mean simply body weight? Overall size? Strength? Each of these can be affected by working out, but the type of workout done will emphasize one of the aspects more than others.
The ads can be fairly accurate, but they generally go strictly by the scale. If you are very intense and religious about your workouts, pay strict attention to diet and recovery, it is feasible to gain 10 lbs in one month. Whether or not that is all muscle mass, however, is where the claims start to get sketchy. Simply because you gained 10 lbs over the course of a month is no guarantee that it is all muscle mass, but the adverts will claim that it is.
Best thing to do is form a picture of what you want to look like, and plan out how you are going to get there. Have a 6 month, 1 year and 5 year goal plan, and reassess/adapt every 6 months or so. Let the mirror and the scale be your guide. Supplements such as protein powders, creatine may or may not help depending on your diet and other personal fitness habits.