Complex sentence: Jennings has been researching the history of Southern California's theme parks for over 25 years and during that time he became good friends with members of the Knott family and long-time Knott's Berry Farm employees.
Split into simpler sentences: Jennings has been researching the history of Southern California's theme parks for over 25 years. During that time, he became good friends with members of the Knott family and long-time Knott's Berry Farm employees.