I will relate our experience.
We stayed at Custer State Park and drove to Badlands from the west.
We didn't take the interstate, but a back road that came into the park on a gravel road.
Unlike the paved "scenic drive" in the park which is crowded with cars, there were almost no cars on the gravel road. We saw bison, bighorn sheep, pronghorn antelope, all from close range.
Once we got to the paved road, all we saw was lots of cars, and of course, prairie dogs.
You can easily spend an entire day, or longer, especially if you get out and walk trails, stop at the visitor center, or if your kids do a ranger program. Those that say it takes an hour or two simply drive the paved road and seldom,if ever, get out of their car.