It's working from the first dose, people report the effects of the strength gain in 3-7 days. This is based on you being the type of person to push yourself in the gym eg drop sets etc then realizing hey i have never been able to push that weight out for that many reps and recover so fast.
If you are the type of person that turns up does his 5-10 sets and doesn't push yourself it's unlikely you will notice much...although it is working...you are reducing myostatin and increasing muscle cell mitochondria.