Fire Wall
Technically there is no wake turbulence timing between two successive HEAVIES. Just sufficient distance to ensure that the aircraft are not closer than 4NM apart (with no closing) when the second aircraft gets airborne. However, here in Hong Kong - due to the fact that all SID departures are running for a considerable distance on the same track, and the DEPARTURES radar controller has little or no room to move, then we have our own in house ruling that we need 90 seconds (generally) between successive HEAVY departures. Once again, this is very much a generalisation. Under some circumstances we provide 2, and even 3 minutes separation for certain sets of HEAVIES. Normally it can be somewhat difficult to get much less than 90 secs between two large aircraft due to the fairly ponderous nature they exhibit in entering the runway and then taking off. Let's be honest, the aircraft doesn't roll until the crew advance the throttles (and if necessary - release brakes).