Well, first radar as opposed to tower people. Should have said Center controllers.
But I seem to remember from another life that in a non-radar environment, you could discontinue vertical separation ten minutes after aircraft on converging tracks had passed each other, just can't remember how we actually calculated that 'meet time'.
2s - If both a/c have a G/S of 8 miles per min, then you double it because they are closing on each other at 16. And if you know the distance between them, you divide the dist. by the speed and get the time at which they will 'meet'. Or do I have it all wrong?