https://isogg.org/wiki/How_long_is_a_generation%3F_Science_provides_an_answer
“At the usually accepted value of four generations per century, ten generations would place the common ancestor only 250 years in the past, in the mid-18th century, suggesting a further search in records of that period for evidence pointing toward the relationship. However, the longer three-generation per century interval indicated by the recent research would place the common ancestor in the late 1600s, with a much reduced chance of finding further documentary evidence bearing on the relationship.”