Most internet plans I’ve seen have a 1TB (or no) data cap which is plenty for most households to stream TV IME. We have a 1TB data cap, and we’re usually around 600GB despite using our connection a LOT.
1TB should be enougn, but one key factor is HD vs SD programming. IIRC Netflix said HD used 3GB per hour of streaming (SD 0.7GB). That covers around 10 hours a day of HD. That should more than cover most needs, but I have a neighbor that turns on her TV when she gets up and leaves it on all day.