Going by what I was taught in home economy, that breakfast should be eaten from moment one wake up before 8pm. And if it exceeds this hour, it is no longer appropriate for the purpose it is meant for. Ideally, the main reason for this is because breakfast is often said to be the most important meal of the day, because during the night hours all the food one ate before going to bed must've been used up for the necessary body functioning before the next morning. And so, it is considered apt for there to be immediate replenishment via eating of breakfast instantly after wake up.
As for me, this rule doesn't really work because I only eat when I am hungry, that's after emptying my bowel. Meaning that if it's morning my system has not made me to visit the loo, I will not eat breakfast until I'll do so, even if it's noon.