Could be within MINUTES........as soon as coolant flow is interrupted 'overheating' can begin.
The coolant is what absorbs the heat....without it flowing the coolant can get overheated but more importantly the tubing overheats cause the coolant is what keeps tubing from getting too hot
Overheated coolant (sodium chromate) turns to crystals that plate out of tubing and obstructs flow of coolant---more overheating/more plating etc
Overheating of tubing causes metal fatigue which results in leaks
Overheating damage is ACCUMULATIVE and PERMANENT