Even on a tight schedule, Supreme Court of the United States earns the hours.
Allow 45 min–1 hr.
You can walk into the highest court in the land for free, see the great hall and (when the courtroom's open) sit for a docent lecture in the chamber itself. The honest catch: there are no guided tours and the courtroom is only viewable when the Court isn't sitting, so what you see varies a lot by day.