Just as a professional follow-on to the joke, a lot of the places I've worked defined it between 50-100 years. Archeologists also have obligations to publish their findings, follow ethics guidelines, work with landowner/government/descendent approval, and attempt to avoid unnecessary excavations entirely.
Only 250 years? I can trace back my ancestors to the 1500s, and I know where they are buried. I have books of stories of many of them. They are real people to me. I’d hate to think of them being dug up to date someone’s curiosity. They are my family.