As ATCO2 says, there may be some national regulations playing a part in this but the rest of his comments are generally what I would call guidance.
Ideally landing clearance should be given before 2 NM from touchdown for obvious reasons but in certain circumstances (nice day, pilot advised of late clearance and given all other relevant info etc.) it can be given as the aircraft crosses the THR.
Landing clearance could also be passed to the pilot by approach - there are times when the aircraft might only be transferred to TWR after it has landed.
As far as international ATC rules/standards are concerned, there is no defined point at which the aircraft must be transferred to TWR - it all depends on the circumstances.
But transferring an aircraft at 800ft (at a little more than 2 and 1/2 miles from the THR) is not the end of the world - although if I wee doing approach I would try and get the landing clearance for the aircraft before the transfer of comms just to make life a bit easier for the crew.