Is there any evidence that it was ever the standard burial depth in practice? I'm aware of the expression and the fact that Daniel Defoe cited it in his fiction, but I don't know that it was ever a legal requirement or custom of burial. In many cases the terrain would make it impractical to dig that far down.

The first reference I could find regarding this standard comes from "Orders conceived and published by the Lord Major and aldermen of the city of London, concerning the infection of the plague" in or about 1665.

Modern burial laws vary widely by jurisdiction, however. For instance, all Pennsylvania law says is that if the casket is encased in a vault, the top of the vault must be at least 18 inches below the surface. If it's buried without a vault or outer container, the casket must be at least 2 feet deep.

As a side question, is 6 feet really the standard in the west, or do countries with the metric system use a different measurement?