the defrost cycle should last no more than 20 minutes, there is a timer for it - and once that timer goes bad, this happens.
The timer should be in the refrigerator cabinet near the thermostat. you will need to remove the thermostat panel to see it. Its a mechanical clock - which should have a screw that can manually rotate the clock. Once the compressor goes off - try advancing the clock after 20 minutes to see if it switches on.
if your run capacitor is missing from the circuit the compressor will overheat from the higher amp draw and trip the overload. Make sure the cap is in place.
Im thinking that your timer is functioning but your compressor is switching off due to the O/L - check the wiring diagram and use all the parts listed on it.