If its medium behind the heavy, then the separation that is applied is due to wake turbulence - so a minimum of 2 minutes. This cannot be reduced by using 5 miles in trail and that is for departure sep where WT isn't an issue.
If a captain wanted more than 2 minutes behind a departing aircraft of a higher category than their own (except S following M) then they should request with the tower - preferably before lining up!