Long-term durability is becoming increasingly relevant for capacitive deionization (CDI) as the technology emerges on the commercial scale. Short-term deionization studies have suggested that Faradaic leakages could be a major factor in electrode degradation, but the long-term effects are still unclear. In this study, we probe the degradation process of the desalination efficiency in commercial CDI modules for up to 52 days of non-stop operation. This corresponds to a little more than 100 m3 of water treated, and the lifetime production volume of the modules is estimated between 150,000–250,000 L of purified water. Surprisingly, the results demonstrate that the absolute long-term loss is largely linear with the cumulative charge leakage. This suggests short-term leakage currents could reasonably predict long-term degradation. Interestingly, the absolute loss mechanisms mean devices with higher total capacitance are more degradation resistant. Finally, shortening cycle times and other methods of reducing leakage would lead to a proportionally longer lifetime. Notably, the first 2 min of the 10 min operation retained 50 % of the performance with only 10 % of the leakage (10-fold reduction). In conclusion, the work provides a method for understanding, predicting, and reducing degradation in long-term operations with commercial CDI modules.