neilr - the best word of advice I ever picked up in this regard was "three consecutive safe circuits".Errors made during the circuit, but corrected nicely, not a problem - it demonstrates knowledge, understanding and maturity.Errors made and not corrected - bad news. Reset the counter and look for three more, consecutive, safe circuits.....then (all things being equal) get out and let them go.Consider discussing with them first what they would do in the event that the aircraft landing ahead of them blocks the runway and closes the airfield, or what they do if ATC said..."whatever".
edit: apologies for the lack of formatting - I did put it there, the site is playing up again.