It only flows if it's in a liquid state. That happens when the temperature gets high enough. Over exceedingly long timespans Sol will enter its red giant phase & engulf the Earth in its outer atmosphere. This will raise all glass remaining on the surface above said glass's melting point. So if you're being exceedingly pedantic & considering multi-billion-year timespans, then most glass will eventually flow. Some glass may escape this fate by no longer being on the surface of the Earth.
Isn't it always in a liquid state? It just has an exceedingly high viscosity at human compatible temperatures. So high that it turns out there's effectively no movement over thousands or even millions of years.
But it's not a proper solid, ie the usual phase transition is absent.
