Exactly how long does it take to turn diamond into graphite?

Diamond is less stable than graphite, so it'll spontaneously turn into graphite, although it takes a long long longgg time. But exactly how long (in years)? Can it be calculated?|||I have not run across an actual speculation. All I know is that its a very long time because of the high activation energy. But as with any geologic process its hard to say "exactly" how long anything takes.|||I thought that they were on the opposite sides of the hard test scale? Yes they are both carbon, but also different structurally.