What is the ruling on saying to someone, "May you live for a thousand years" or "May you live for a million years," and does this reach the level of disbelief if it is forbidden, and what should be done if this is said?

These words are said to exaggerate in supplicating for a long life, and their intent is for the supplicated-for to remain alive for the longest possible duration. This is not forbidden unless their continued existence poses harm to Islam and Muslims. It is, however, desirable to qualify this supplication by stipulating adherence to obedience to Allah, so one would say: "May Allah keep you and prolong your life in His obedience." You should qualify the supplication for yourself for a long life by stipulating that it be in obedience to Allah, and supplicate for the one who supplicates for you in the same manner, qualified by obedience.