We as controllers (at least in the US) have rules that allow multiple airplanes on the runway at the same time. Category I Aircraft (Single engine props, C172, P28A, SR22, etc) behind Category I must be 3,000 feet and airborne before the arrival crosses the threshold
Category II (Twin Engine prop PA44, C402, etc) behind a Category II must be 4,500' and airborne before the arrival crosses the threshold
If ANY aircraft is a Category III (All others. Jets, some King Airs, AT72, DHC8, etc), the aircraft must be 6,000' and airborne before the arrival crosses the threshold.
The key to understanding this is the trailing aircraft. IF a CAT I follows a CAT II, you only need 3,000' and airborne.
Arrival to arrival: CAT I: 3,000'
CAT II: 4,500'
CAT III: Clear of Runway
Again, the trailing aircraft is the determining factor. However, if ANY of the involved aircraft are CAT III, the arrival must be clear of the runway.
It is good practice of a controller when things get tight to tell the pilot on short final that runway spacing is good so they don't have to go around.