The dominant theory is that he should get ten minutes for every grade. So, in his third year of school, he's at a half an hour. By the time he is in extracurricular activities, experimenting with dating (scary thought) and working his first job, he will be at two hours a day.
Homework is often justified with the "real world" argument. When I point out that kids should have the freedom to learn independently, play around with friends and socialize in their free time, I hear the rationale that in the real world they will have to work hard whether they want to or not.
In the real world, extra hours will get overtime and people get to choose their jobs. Only in the most authoritarian of nations do we expect people to work additional hours in jobs that are forced upon them by law.
Homework is often justified with the excuse that "the homework is intense in college." True, but so is the free time. And in college, students have freedom of space, freedom of choice and additional responsibilities that come with adulthood.
So, if you want to make homework like the real world, it needs to be extra credit and it needs to be based upon student choice. If you care deeply about mirroring school and the real world, you need less coercion, more freedom, more choice and more self-directed responsibility.