If you have an hour, spend your time in the visitor center. You can see over 80 fossils, two video programs and fossil preparation demonstrations. You'd also have time to become a Junior or Senior Ranger.
If you have two hours, spend the first at the visitor center. Then consider hiking one of the interpretive trails, take the scenic drive, or participate in a ranger program.
If you have three to four hours, you have plenty of time to check out the visitor center and scenic drive, hike one or both trails, and become a Junior or Senior Ranger.