22 year old male, taking finasteride since Nov 9th. How do I know when it's going to work for me? (Photo)

My hair is Shedding a lot more, i noticed maybe because i haven't cut it in a while, but its also extremely itchy and burning from time to time, how do i know when its going to work for me? and how would i know if i do have MPB, and not something else i walked into the dermatologists and left after 5 minutes he didn't even test my hair or do anything just looked and said u have MPB and here is your medicine