Highly stable a-Si TFTs reported recently with extremely long operating lifetimes under DC gate bias are attractive for analog drivers of the OLEDs in AMOLED displays . At room temperature, the time for the DC saturation current to drop to 50% is predicted to be 100 to 1,000 years. However, the lifetimes were extrapolated with a stretched-exponential model for defect creation in a-Si, based on only month-long room temperature tests. In this study, we present a two-stage threshold voltage shift model for lifetime prediction from temperature dependent measurements. We find that (i) a unified stretched exponential fit models the drain current degradation from 60°C to 140°; and (ii) there is a second instability mechanism that initially dominates up to hours or days at low temperatures, so that tests conducted only at room temperature may not predict lifetime accurately.