There are lots of separation standards used around the world depending on equipment and circumstances. We use 3, 5, 8 and 16nm in our airspace, depending on which radars we are using and what mode of radar display is being used. But you'd be a brave/stupid controller to use 5nm behind an A380 or B757 in the cruise; never forget wake turbulence separation. You'll get complaints from some pilots even when you are applying the required minimum separation if they feel the bumps.