In Minimum separation between a landing aircraft and another crossing its runway? it was explained that once an aircraft has clearance to land, that runway is reserved for the exclusive use of that aircraft.
At busy airports you can usually see a long line of four or more airliners descending a glideslope towards a runway.
It seems as if, by the time the previous aircraft has turned off the runway, the following aircraft has a very short time to obtain clearance or decide to go around.
At what point is each aircraft given clearance to land by ATC?