Oh I see. I would think a good criterion is that all their immediate and even second order family members would have to be dead. Hits too close to home otherwise. So... maybe 100 years?
This guy below wasn't even a skeleton- his body was found mummified and still surprisingly well preserved, frozen in the Swiss Alps, and dated back to... 5000 years ago (dawn of the copper age)! Forensics found that his hand had multiple semi-fresh scars, as if he has been in a recent scuffle a few days before. They also found a stone arrowhead lodged in one shoulder, which had unfortunately hit a major artery- the proximate cause of death. He had some stone tools, but a copper ax on his belt- sort of the technological equivalent of the latest version of the iPhone of the time- suggesting he was probably someone very wealthy or higher ranking in society.
So in trying to recreate the crime, they are thinking he was probably in an altercation a few days before, somewhat injured in the scuffle, and was trying to escape through the mountain passes when his assailants caught up to him and finished him off with an arrow shot to the back, leaving him to die in the ice and snow. If only skeletons could talk. Stone age drama!
en.wikipedia.org
Now I'm thinking most of his immediate descendants are dead by now, and so it's OK to display him in the museum.