PCG Graph in PCG Volume: when does it update

So I’m finally getting the hang of doing what I set out to do in the PCG graph. I have a hacky lame way to do what I need, and a way involving some powers and square roots. In materials those are expensive operations. My question is, in game, when does the pcg graph run? Is this something that chugs away on tick, or when some variable changes, or is it on begin play?