After i have been played for some days with various vertex and fragments shaders seems clear to me that this programs are called by the GPU at every and each rendering cycle, the problem is that I can't really quantify this frequency and I can't tell if is based on some default values or not because I don't have a big collection of hardware right now to do extensive tests.

For what i know the answer could be really trivial like "it's the same of the refresh rate of your monitor", but i would like some good answers on that to be clear on this.

For instance looks really odd to me that all the techniques used to control the amount of FPS that i have seen until now uses a call for the OpenGL function glutGet(GLUT_ELAPSED_TIME) to retrieve a value in ms about when the rendering started but I have to relies on the CPU to do the math. Why I can't set an FPS value in OpenGL if OpenGL clearly has a counter and a timer/clock?

PS I'm referring to OpenGL 3.0+