Since the invention of cable, many TV shows have stuck to their allocated time, approximately 40 minutes or under an hour, for drama and sci-fi. As for sitcoms, 20-30 minutes has always been deemed adequate. Such time allocations are determined by various factors, such as advertising and tight programming schedules, among other things.
Source：https://www.cbr.com/best-feature-length-tv-show-episodes-ranked/
