The thermal stability of sintered NdFeB magnets can be described by the temperature coefficient of remanent flux density α only on condition that demagnetization due to rising temperature or field and oxidation of the magnets are totally avoided. This also requires the control of time-dependent demagnetization at elevated temperatures. We have studied the time-dependent polarization losses in sintered NdFeB magnets with different coercivities by determining temperature T0 for each magnet (Pc=1.1). T0 is the maximum temperature at which the total polarization loss even after 30 years of exposure is estimated to be less than 2%. Temperature T0 for four magnets produced from different materials are presented in this work. Flux losses at temperatures higher than T0 after different exposure times are also described. The estimated flux loss after 30 years is found to be roughly twice the loss after 1 h, except in the vicinity of T0. At temperature T0+10°C , it could be as high as four times the loss after 1 h.