Nowadays, post-tensioning of glass beams in the style of prestressed concrete is a promising option for advanced architecture but lacks in vital knowledge. Previous research has shown that prestressing creates fail-safe and even ductile structural glass beams. However, long-term effects, namely losses of prestress in post-tensioned glass beams, have not been investigated yet. This paper introduces a general design concept and suggests an analytic model to estimate losses. As glass is known not to show considerable creeping deformation over time, the model focuses on the contribution by plastic parts of structural glass elements. The results include a proposal to use creep coefficients for the interlayer in laminated glass as well as the blocking materials used in contact structures. This approach is advantageous due to its ability to be applied with arbitrary material models, thus broadening the application to a variety of interlayer foils and advanced glass contact materials. A comprehensive experimental investigation was carried out to test the hypothesis using 28 specimens in a variety of tendon diameters and cables forces as well as reference specimens. The tests make it possible to describe losses analytically using regression functions. Moreover, the investigation revealed additional effects that need to be included in a model for life-time prediction. The given results show the time-dependent effects on novel precompressed glass beams for the first time and build a basis for future structural options in facade design and glass roofs.