From God's persepctive, it is too late only once a person dies. As Ecclesiastes 11:3 says, 'In the place where the tree falls, there it shall lie.' From man's perspective, it is too late once a person no longer has any willingness to come back to God. In other words, God is always willing for forgive and restore, but sometimes people stop being interested in seeking forgiveness and restoration.
Jesus explained it this way, 'And this is the condemnation, that the light has come into the world, and men loved darkness rather than light, because their deeds were evil. For everyone practicing evil hates the light and does not come to the light, lest his deeds should be exposed' (John 3:19-20). The light is available to save everyone, but because some people love darkness, they refuse to come to the light to receive salvation. This is their choice, which means that it is only too late once a person has made the final decision to remain in darkness. Once this happens, they enter a place of deception and their end is sealed.
We also suggest you read our article titled 'What is the Unforgivable Sin?' found on the Traditions of Men shelf of the Apostolic Free Library.