To determine how much lean muscle mass you can put on in a given period needs stats. Exactly how much have you put on since you started training? What is the size increase for major muscles; pecs, thighs and biceps?
You need a baseline so you can accurately predict potential growth. You keep detailed records don't you; weight weekly, sizes monthly?
Do not be impatient. Tennis players and Aussie rules footballers have impressive physiques but it takes time.
I am reassured you know who David Laid is. Quite a transformation. I give him the benefit of the doubt - very good genetics and lots of hard work rather than steroids.