The degradation predicted by classical DC reliability methods, such as bias temperature instability (BTI) and hot carrier injection (HCI), might not translate sufficiently to the AC conditions, which are relevant on the circuit level. The direct analysis of circuit level reliability is therefore an essential task for hardware qualification in the near future. Ring oscillators (RO) offer a good model system, where both BTI and HCI contribute to the degradation. In this work, it is qualitatively shown that the additional off-state stress plays a crucial role at very high stress voltages, beyond upper usage boundaries. To yield an accurate RO lifetime prediction a frequency measurement setup with high resolution is used, which can resolve small changes in frequency during stress near operation conditions. An ACDC conversion model is developed predicting the resulting frequency change based on DC input data. From the extrapolation to 10 years of circuit lifetime the model predicts a very low frequency degradation below 0.2% under nominal operation conditions, where the off-state has a minor influence.