In genealogy, the length of a generation in the past has been used principally as a check on the credibility of evidence — too long a span between parent and child, especially in a maternal line, has been reason to go back and take a more careful look at whether the received information reflects the actual reality, or whether a generation has been omitted or data for two different individuals attributed to the same person. For that purpose, the accepted 25-year average has worked quite acceptably, and birth dates too far out of line with it are properly suspect. Therefore, a 3rd generation in this case is 75-years, give or take, which in my mind is that old white guy who still hates Natives.