Is it even possible to practice more than five hours a day? After doing all that practice you should be pretty tired and needing of some rest. If by you do by some miracle have the power to continue the quality of the practice is not going to be high. It should then be better to go and do something else.
Also the amount of time is of secondary importance to the quality of the practice. Three hours of high quality practice is definitely better than five hours of doodling.
(2) For the musicians out there: how does Mr. Perlman's advice align
with your experience?
This musician can tell you he is correct. Five hours with then minutes breaks in between the hours is absolutely the maximum you can do in a day. If you do that for two or three years your progress will be immense.
I also think that for the long term motivation of the student it should be better to not have a extreme practice schedule. It is also worth noting that these five hours a day philosophy is and should be directed at students at a Collegiate level of musicianship.
For young children it is often more important that there be a emphasis quality over quantity. For them a half hour a day under a parents supervision is better. When the hit there teenage years and they can start to motivate themselves then the practice becomes longer.
You should also take note that Julliard is solely motivated to produce performers. It is entirely possible to study for four years there and not know how to explain to a person how you get the flats in Ab minor.
In a music academy where you may want to learn to become a teacher there may be less practicing to focus on certain other parts of being a teacher like methodology, psychology and theory.
It all depends on who you teach, what level they are and what there ambitions are.