30 minutes would be pretty quick, but you don't need long. Definitely take the boardwalk trail that goes to the base of the mountain, and loops back around to the studio. Nice views up George Washington's nostrils and nice views of Black Hills and surrounding badlands. Take a little time looking at the interpretive exhibits.

But make sure you make time to visit Wind Cave National Park (take the cave tour), Jewel Cave NM, Custer State Park (including the crazy windy roads, Crazy Horse, Badlands National Park (about an hour drive past Rapid City), and Wall Drug. Go to Deadwood as well.

And check out Devils Tower if you have time to take the couple-hour drive to the west.

The Black Hills are one of the great areas to visit in the United States.