How do I know it's time to stop breastfeeding?

It's a personal choice. Breastfeeding is a relationship that's forged between the Mother and Child. So unless either party wants out, the breastfeeding relationship will continue. But having said that, if it's getting burdensome such that it's affecting your sanity or health, stopping is an option for consideration. At the end of the day, Happy Mummy = Happy Baby