Every time a shader is used basically. So you need to actually do the things to make it happen. People are playing, and building, but a game of this size there won't be a real, truly complete shader cache for a long while. I downloaded a cache with 17k shaders, started playing, and in the first battle was getting a lot of stutters simply due to the fact that my playstyle was apparently so different from the person who originally generated all these shaders. I was doing stuff they hadn't and causing shaders they never encountered to generate. Its a great starting point to get a preloaded one, then just generate more as you play and the game will get smoother and smoother as you go.