Our experience was similar to what THOM02099 has said. Yes, you can certainly see Rushmore in less than an hour. Heck, you can see it in 15 minutes. But for our family, we wanted to know about the monument, how it was built, what was the history. If you are curious at all, you can spend considerable enjoyable time finding out info and truly experiencing an American marvel and salute to great American presidents.
I had a good friend who was a park ranger at Everglades National Park. He said that many visitors would be perfectly happy if they had an alligator chained up outside the visitor center. The family would pile out of the car. The dad would say "OK kids, we've seen the gator, now let's go to Orlando and Disney World. "
If some people's idea of visiting national memorials, national monuments and national parks is like that family, I suppose it is good, because those of us who truly appreciate these national treasures will have a less crowded experience.