Long-term measurement of bias temperature instability (BTI) degradation obtained from an on-chip sensor is presented. The sensor reports measurements periodically with a digital output. Implemented on IBM's z196 enterprise systems using IBM 45 nm technology, it can be used to monitor long-term degradation under real-use conditions. Over 700 days worth of ring oscillator degradation data from customer systems is presented. The data obtained by this sensor are consistent with models based on accelerated testing.