It depends on the circumstances.
In a nut shell:
On approach, Heavy to Heavy should be 4 miles (minimum)
On departure, 1minute between the rotations of the two Heavy aircraft (that being the point at which vortex begins).
There is no wake vortex spacing requirement when a Heavy is following a medium or a light. In other words, standard IFR separations apply or visual if circumstances permit. Normal IFR separation would be 3 miles but if departing on the same route it would be 5 miles unless there were an issue with relative speeds.
Point 4
