The life time of a brushed motor depends on several factors like quality, max. RPMs and personal abuse.
Some manufacturers tell the life span of their motors under normal conditions.
As a rule of thumb: the higher the RPMs, the lower the life expectancy. There are motors with 10-12k RPM that last 10-15 hours while others with 15-17k are tagged 5-6 hours.
Bearings and of course the brushes are prone to wear&tear - the more you crash the faster they will go down the drain.
High pitch or crackling noise are an indicator for this and you can feel the play or scratching when you move the shaft.
Also a loss of power compared to fresh motors points at the beginning of the end.
Cheap motors with cheap bearings will be worn out much faster than quality motors which often cost double or 3 times the amount of the cheap ones.
So in summary: look out for untypical noise, play and loss of power bf swapping them. If 1 motor starts to act up it´ll only be a matter of time when the others will follow. It´s better to change all 4 motors then and keep the others as spares for upcoming crash events.
Norbert