"On the first day of my first stained glass course, my instructor attempted to perpetuate the common myth that glass in windows flows over time, that's why glass is thicker at the bottom in church windows."
A few years ago, Edgar Dutra Zanotto published an article in the American Journal of Physics (#66, May 1998) in which he calculates the time it takes for glass to flow. He estimates it will take 100000000000000000000000000000000 (10 to the power of 32) years for the glass to show any noticeable change at normal temperatures.
Oh, by the way, the age of the universe is 10 to the power of 10.