I know part of this answer will be.. "it depends." A rank newbie, underweight, 20 year old male can probably put on a ton of muscle fast, so there are really no holds barred for such an individual. But say a 35 year old intermediate? I would think no more than 2 pounds/month of lean mass gain, which would make 1 pound/week a good guideline to bulk on...2 pounds of fat gained for every 2 pounds of muscle, in 4 weeks lets say. Any thoughts on this?