Propellor,
If both depart from the same point, then if it's, say, a Heavy 747 followed by a Medium 737, then it's 2 minutes. If the 737 departs from an intersection, then it's 3 minutes. However, at Heathrow it's a bit more complicated, because some of our intersections, 102 on 09R, 86 on 27L and 18 on 27R, are not considered as intersections for vortex purposes.
You mean if a 747 took off from, say runway 18, then a 737 took off from runway 36? Got no idea! The thing you'd have to watch out for is the crossing of airborne tracks.
Gonzo.