Here we use radar separation on departure (except behind an A380) and a Medium would be 5 miles behind a Heavy at the time the Medium becomes airborne. A wake turbulence cautionary shall also be issued.
Take-off clearance to the Medium in light winds would probably be issued when the Heavy is 4 miles away from the threshold or about 70 seconds after rotation of the Heavy. In stronger winds it takes longer to achieve the spacing, so it would be nearer the ICAO 2 minute standard to achieve the 5 miles.
If you need more than 5 miles (i.e. 2 minutes) please inform ATC before you accept the line-up clearance.