Depends on the vehicle...in my diesel 3/4 ton PU truck, under 2k rpm @ 70 mph. In my old MB diesel, 3500 rpm at the same speed, (more or less). In my Jetta TDi, maybe 1200 rpm in 6th gear on the freeway? Can't remember that one, I'll report back when I drive it.
I used to know a guy with a 911, (mid-year car), who drove around in second gear at 5k rpm. I asked him wtf was he doing and he said that he had been told to "keep the revs up." He did not know why, of course. I explained to him that he was not driving a 1957 4-cam Carrera or similar old sports car where fouled plugs were an issue from primitive bad fuel mixture systems and that the thing between the seats was a gear shift and that he should give it more use. He was screaming around town, wasting fuel with absolutely zero performance benefit...he was way out of the torque band even if he was on a track.
