Pure muscle mass, for the average beginner, probably between 12 to 16 weeks. Of course the basic program should be heavy compound exercises. Like squats (breathing version preferred), dips/benches, row, cleans,etc. Our athletes do this on a regular bases. Usually 3 or 4 compound exercise, with 2 to 3 short and to the point workouts. Really not rocket science gaining muscle.
If your an experience lifer and near your muscular prime, than a little more complex planning will have to be considered. Diet and a training program designed for your body type alone may be number 1. Even if all this is taken in consideration, that added PURE muscle may take many months or even years. 20lbs gain of quality muscle is quite a lot for an veteran BB'er.
If really desperate or mentally deranged , and don't care about side effects (now or near future health) than get to know your local dealer ( personal view: I have never favored the drugie approach). Just to note that the family of 'roids do not work on everyone the same way. Some can gain rapid muscle mass/strength and others not that much for their efforts. Even if designer stacks are applied. First clue, might be , that if born with good genetics, gains will come fast. Poor genetics, not so much. No matter if using or a natural. Good Luck.