From Erwin Chargaff’s (1986) Serious Questions (Soon at IWP Books):
I know many people who, besides listening to the radio, spend at least three hours a day before the television set. That means that they spend nearly one month and a half in a year on this form of amusement. If they do that for fifty years, they have expended six and a quarter years of their lives sitting before that box, imbibing more or less complete rubbish. The argument that most of those people would not have known what else to do is invalid, as we do not know what they could have done. Some of them might have committed murders (they do that anyway, only outside of the prime hours of broadcasting), others might have created masterpieces; all of them would have grown old in greater dignity. Our times have made us all into spectators: we watch us voting for those we have watched, we watch us being governed by lighthearted thoughtless authority as if everybody’s life were a badly put-together serial. The oily, grinning slapdash that fills the screens, claiming to have been elected to govern the people, they all seem in their youth to have been extras in a film depicting the court of Louis XIV.
2022 Update: People spend about 5 hours a day on their smartphones (some surveys say more, some a little less; definitely more for younger people), which if they continue doing for fifty years, they will spend a little more than 10 years of their lives looking at the black mirror, “imbibing more or less complete rubbish.”