Hempy, using the AIP format c:
a. CONTACT (unit call-sign) (frequency..."
b.* (frequency)
c. AT (or OVER) (time or place)
CONTACT (unit call-sign) (frequency...)
Tower uses "WHEN VACATED CONTACT GROUND..."
On the broader point of when to pass the instruction, when the aircraft has commenced the turn or is very slow and approaching the exit taxiway. When the aircraft leaves tower frequency after that is not of great importance. The debate has ignored the fact that runway separation standards do not permit another landing or takeoff until the landing has vacated and is taxiing away. A visual check of the runway is required before all landing and takeoff clearances. In reduced visibility, extra reports or Surface Movement Guidance systems are used.
Once the Aerodrome Controller passes the transfer, he/she has decided that the aircraft is no longer required in frequency.