since there are too many trumpet players out there and not enough jobs i would say for all of you to practice no matter how tired you are, infact pushing in on the lips with the mouthpiece really helps you with high notes!! dont stop untill you cant feel your face!!
In my own practice I try to play things as perfectly as possible. When it becomes clear that I can no longer perform at the expected level due to fatigue I will take a break.
so i would say if the sound or level of performance drops becasue you are tired it is time to take a break or call it quits for the day.