It all depends on genetics. For some men, it can take decades to become a Norwood 5 when balding first started. For some men they can reach a full Norwood 7 within five years after they first started balding.
Totally dependent on the individual. I started losing my hair at 19 and had various periods of rapid loss, stability and even some (mild) regrowth from meds. Overall, my trip to NW5 took about 20 years. I have friends who went from a full head of hair to slick bald in less than a decade, and others who had some loss and stabilized on no meds and never progressed over decades.