While several studies have experimentally investigated the fatigue behavior of polymeric metamaterials, systematic methodologies for predicting their fatigue lifetime are still lacking. Therefore, this paper proposes and experimentally validates a comprehensive methodology for fatigue lifetime prediction of polymeric mechanical metamaterials. The approach is demonstrated on a metamaterial with tunable stiffness manufactured by additive manufacturing from polyamide 12 (PA12). The base PA12 material was thoroughly characterized by mechanical tests, including tension, compression, dynamic excitation, and fatigue loading. The resulting data were used to develop a material model for finite element simulations, enabling stress analysis and identification of fatigue-critical locations within the metamaterial structure. The numerical model was validated experimentally under compressive loading. The influence of printing repeatability was also investigated, demonstrating that relatively small printer-dependent geometric deviations can significantly affect structural stiffness, dominant failure locations, and fatigue lifetime. Subsequently, fatigue lifetime was estimated at the identified critical locations using four different criteria. The results show that the simplest criterion based on the first principal stress provides the most accurate fatigue lifetime prediction, which was confirmed by experimental fatigue testing of the metamaterial structure.