How long would you continue trastuzumab and pertuzumab in a patient with ER+ HER2+ breast cancer with initially osseous involvement treated with ACT-HP and is now in CR by PET for >2 years?

All physicians who treat metastatic HER2 breast cancer eventually have patients who appear to have been "cured" of their metastatic disease with available agents. This question crops up from time to time, wondering if there is some measure of time or test we can use to determine when it would be rea...