Dashboard Variable Update Frequency

How often are dashboard variables evaluated? Is it only when they're accessed by something in the dash? Presumably that's the case. I'd like to create a variable that adds to an array every time a controller button's pressed, which is why I'm asking. My thinking is that any dash element retrieving an array element  would need to have a minimum refresh interval that's < the amount of time the button controller would be pressed. Otherwise, the button press would be missed.