A general belief among members of the scientific community is that glass articles can be bent irreversibly and that they flow at ambient temperature. This myth is mostly based on widespread stories that stained-glass windows of medieval cathedrals are thicker in the lower parts. In this paper I estimate the time periods required for glass to flow and deform at ordinary temperatures, using calculated viscosity curves for several modern and ancient glass compositions. The conclusion is that window glasses may flow at ambient temperature only over incredibly long times, which exceed the limits of human history.