<<It said three generations after the Final War. The DM also determines how long a generation is. It could be anywhere from 20 to 35 years or more.>>
Generally speaking, a "generation" is considered to be the time it takes for one generation to be born and reproduce. For people, the term usually means about 25-33 years (time to be born, grow up and have one or more kids). The absolute low end for humans would be maybe 15 years per in a high-birthrate, high/early deathrate situation. End result in this case, minimum time for 3 generations would be about 45 yrs; likely time span for 3 generations would be around 75 yrs.