The total electron content of the ionosphere affects the propagation of radio signals used by Global Navigation Satellite Systems (GNSS) to provide positioning, navigation, and timing services. Analysis of extreme total electron content (TEC) values is thus important for assessing space weather hazards for these systems and for the development of risk mitigation strategies. Variability of TEC is produced by regular changes of solar radiation attributed to the solar rotation, to seasonal and yearly variations and the most significant contributions coming from variation over the course of the solar cycle. During space weather events, the observed TEC is a superposition of the periodic background TEC and enhancements caused by the event itself. Based on 20 years of data, spectral and statistical analyses of day-to day TEC variation and absolute TEC values have been undertaken to estimate the input from space weather events to TEC values and to assess the amplitude and frequency of TEC variations expected in the course of a solar cycle. Extreme values of the TEC have been analyzed using a generalized extreme value distribution to determine the 1-in-44 year (4 solar cycles) and 1-in-100 year maximum TEC, separately for the high, mid, and low-latitude regions and for global TEC values. This analysis provides criteria for the development of operational systems and for vulnerability assessment of critical systems dependant on GNSS.