I suppose you could start with the standard rule that the default mode for TV programming in the U.S. is reruns. Some channels show no original programming at all. Starting from that bottom point, you can work upward.
Comedy Central (a “basic cable” channel), usually shows some original programming from 9 p.m. to midnight, at least the half hour of “The Daily Show.”
H.B.O. (a “premium” pay channel), usually shows original programming on Sundays from 8 p.m. to 11 p.m. But their original shows are ordered in limited runs, so maybe there will be non-original programming about half the time.
Other specialty channels run new programming when they have it and reruns when they don’t. Often it’s hard to tell when you’re watching a “new” show. It hardly seems to matter whether a particular episode of your favourite cooking show is new or not.
Oh, I suppose “Emeril Live” is always new (daily at 8 p.m.), but, I mean, what a waste.
I don’t know if they still do it, but for some time, the T.L.C. channel was showing two new hours of “Trading Spaces” every damn Saturday of the year, so that was a reliable spot for non-rerun programming.
The programming for broadcast channels (“real TV” before cable or satellite) are split between the local stations (who own the frequencies) and their affiliated networks. For many local stations, their only original program is news, which they do, say, three or four times a day in various formats. The 7-8 p.m. block is usually given to reruns or syndicated game shows/Hollywood gossip shows. The block between 8-11 p.m. is usually given to the networks and they use those slots for original programs, say 25-30 weeks out of the year, filling the rest with reruns. The late night slots are usually given to shows that have no shelf life, so they are not usually reruns. The rest of the daily block can be filled with reruns, original syndicated programming (usually chat shows or “People’s Court”-type stuff or game shows), or network programming (soaps).
If I’d hazard a guess, I’d say that at any one time maybe 90 percent or more of what’s on TV has been shown before.