You may want to check out “The Zone” by Barry Sears. You should be able to get it at the library. In chapter five he relates a 6 week training program for elite football and basketball players. The athletes gained 11 pounds over the time period, increase LBM 8%, and dropped body fat 20%. I would estimate from these numbers about 1.5 to 2 pounds of muscles were gained per week. I am assuming these athletes were big and genetically gifted. They also were training hard and being meticulous about their diet. I suspect what they were experiencing was not normal.
Covert Bailey in his book “Smart Exercise” claims the most muscle mass a person can add in one week is a pound. (page 234)
Dr. Fred Hatfield, in his book “Hardcore Bodybuilding: A Scientific Approach” includes a chart on pages 179 that lists the amount of time it will take to put on a given amount of muscle mass. For someone with a lean body mass of 300 pounds it figures out to about a pounds a week. People with a lean body mass of 100 pounds figure out to about a half pound a week. He also gives ranges in 20 pounds intervals from 100-300 pounds lean body mass. All the figures fall between the half to one pound figures.
From this data I would put the amount of mass a person can put on with dedicated work, proper diet, and good rest between half to two pounds a week. Putting on muscles is a slow, steady process. The numbers may be skewed for those who eat massive amounts of protein and are juiced, but for most people putting on 25-50 pounds of muscle in a year is a major accomplishment. Just my thoughts.