via press release from nielsenwire:
Traditional primetime is eight to 11 o’clock at night, Monday through Friday, but Nielsen finds that more Americans tune in from 9:15pm to 9:30pm than any other period during primetime. The tail end of primetime—10:45 to 11:00pm—is when the fewest viewers use their televisions.
Although there are no differences between when men and women watch TV during primetime, age is a factor. Viewers ages 18-49 typically tune in later. For this demographic, 9:45 to 10:00pm is prime viewing time, while they use TV the least at the beginning of primetime, from 8:00 to 8:15pm.