"For those with experience, how much time would you spend at The Badlands, Mt. Rushmore, Wind Cave, Crazy Horse and Devils Tower?"
We camped at Badlands for a couple of days. It is as the name implies, beautiful, but bad. In my opinion the others are day (or less) visits. Put Custer State Park on your list. Jewel Cave is awesome too. There are just too many neat spots to stop and get to know people on the way. Cody, Cheyenne, Jackson, etc.,etc.
Don't short change yourself by rushing to a schedule.
I'm sure you have heard that Yellowstone can be a challenge during peak months, so I would get there before if you can.
Lots to read on past posts about all.
Max