It is impossible to provide a simple answer to this question, as every couple and person is different.
Unless a break-up is caused by some outside event, there is rarely one single point in time when you know that the relationship is over. Typically, it is something you feel over time.
There could be changes in your emotions towards the other person, or you sense a drastic change in their behavior, or you develop unshakeable questions about the relationship. You or your partner just needs to say it out loud.