Good question, actually. At least two hundred years, would be my estimate - long enough for most people who would know the person to no longer be around.
Or if you're an archeologist from the 19th and early 20th century, any time is archeology long as the grave you're robbing isn't from anyone you know.
UN:ThreeCoffins
Or if you're an archeologist from the 19th and early 20th century, any time is archeology long as the grave you're robbing isn't from anyone you know.