How often is the update function called ?

I have a variable, which gets changed every update. Now I want to know how often update gets called per second.

Thank you for your answers.

It is the fps. You can get it with :

void OnGUI()
    GUI.Box(new Rect(0,0,100,50), (1/Time.deltaTime).ToString())

If you have v-sync enabled then it will also depend on your graphic card

For instance you will probably get about 60 to 70Hz which is the rate of your GPU.
But my computer drops to 15 Hz if I use power saver.