The Lithuanian national standard of voltage is maintained as the basis for calibration and measurement capabilities of Lithuania published in the Key Comparison Database of the International Bureau of Weights and Measures (BIPM). The stability and uncertainty of the voltage value measurements, performed since 2004 using the calibrated values of the Zener solid-state voltage standards (zeners) to predict their future behavior, are discussed. Conclusions regarding short- and long-term predictability of their behavior, which can be used for choosing an appropriate calibration period, are presented. An estimate of merit for approximations is proposed; based upon the estimate, it is concluded that the hyperbolic approximation is the best one in the most of the cases. Also discussed is the behavior of voltage dividers used in the zeners as well as the recovery of the zeners after a failure of their power supply. It is concluded that the voltage standards operated by the Lithuanian National Electrical Standards Laboratory feature stable drift of the voltage reproduced, which is well predictable by means of linear or non-linear regression.