Really interesting discussion - dog will be fine,in the parking Garage, as long as it is not near 100, JMHO. 
For the time to see the monument -- I have been probably been to it a dozen times and never spent just an Hour! Come see it in the daylight, then drive some of the most beautiful drives in the country, Come back and see it at night --GOOSEBUMPS! Then the other mountain, Crazy Horse, and Needles, Lake Sylvan couple of Caves,just relax and for us usually three days - a week before we can even think about leaving.
While I am more of a fan of the natural beauty-this area and Mt Rushmore is just Amazing. I have never been to those rocks that I did not feel like the time I visited was just too short. If you ever need to understand more about the area watch the film North By Northwest, then get out and enjoy the Wild and Beautiful sights. I can't wait for the next trip -- likely with a few of the Granddaughter's.
JMHO,