Wheatwine:
I've lived in the USA all my life, and so only wore the cap and gown on 3 occasions. When I graduated from high school, when I graduated from college, and when I got my masters degree.
That matches my experience, except you left out graduation from junior college, which is a fourth occasion for some. I skipped the middle two occasions because they didn't seem important at the time and just attended my High School ceremony and my Master's graduation, which was some 35 years later! Both times, I made do with a cheap disposable gown.