P0135 Fault Diagnosis on 2002 Chrysler Town & Country - How Long for O2 Sensor Ready Test?

Hello everyone, I'm experiencing a P0135 diagnostic trouble code on my 2002 Chrysler Town & Country. After replacing the upstream oxygen sensor due to a heater circuit fault, I drove the vehicle over 150 miles and reset the ECU. However, the 'Ready' status for the heater O2 sensor remains inactive. I used a scan tool to activate the ASD relay and observed a voltage drop on the upstream O2 sensor signal — this suggests the sensor is receiving power and responding correctly. Still, I'm wondering: how long does it typically take for the ECU to complete its own self-test cycle for the heater oxygen sensor? Is it possible that the system is not performing the test at all? Could this be a faulty ECU? Or could there be an issue with the heater circuit wiring or grounding? I'm looking for input from others who have encountered this P0135 code on their 2002 Town & Country models. Any insights into troubleshooting steps would be greatly appreciated.