This question has two interpretations. In a given session, you should practice until your energy is at its peak. If you are practicing properly, your practice will be energizing, and you will feel your energy and flexibility increasing. At a certain point this peaks, however, and if you practice beyond this peak, you will start to become exhausted. Stop before this point.
In the course of your life, there is no reason to ever stop practicing tai chi. It is an art that you can continue to practice and improve until the day you die.