Significant muscle building takes years rather than months, and the amount of muscular weight increase feasible in a month is actually relatively little. Any significant weight changes over the course of a month are usually the consequence of fluid loss or retention rather than new, bulging muscles. The only exception may be if you are using drugs that have an anabolic effect - for example, steroids. In this case, you might be able to gain some muscle in a single month. The average man's body weighs about half a ton, and the maximum muscle mass that can be sustained for any length of time without losing water content is close to that figure - around 500-600 grams (about 1-1.3 pounds). If your current weight is 90 kg (198 lbs), then it follows that your maximum muscle mass would be about 4500 g (100 pounds). Most men will not reach this level even after many years of training so it is clear that it takes at least several years to grow big muscles. In fact, it has been estimated that it takes more than 10,000 hours of practice to become an expert in a skill such as tennis, golf, or boxing, and it also takes at least five years to develop serious strength levels appropriate to these sports. Thus, if we assume that you make enough effort during this period to keep up with the experts, then you should be able to add significant muscle mass to your frame.