If you are on intermittend ADT you can choose when to start with Lupron shots again, there is no specific PSA value everyone agrees on.
Since I want to extend my vacation als long as possible I use this study protocol as a guide
. Unfortunately the full text is not available for free, so let me cite it here: "The condition for restarting treatment depended on the level of PSA at randomisation. For the patients whose PSA level dropped to <4 ng/ml, therapy was restarted if the PSA level rose to greater than 10 ng/ml for symptomatic individuals or to greater than 20 ng/ml for those without symptoms."
So if you choose to follow that study protocol you could start again with Lupron shots at a PSA value between 10 and 20.