My grandchildren can watch as long as they want AFTER homework and chores are done aND before bedtime. Business before pleasure. They must stop before bedtime. Of course they whine when told to stop.
When they're at my house, I don't pay much attention to screen time. My teen granddaughter lives with me and often reads books on her phone. She doesn't usually use screen time until 6 or later. She is physically active. My teen grandson is in special ed. He doesn't like reading. By looking up his interests on the Internet, he reads. Computers at school are used as a part of learning.
I believe that there are many beneficial ways to use screen time. "Experts suggest limited use. I believe by the time my grandchildren are adults we will be needing to use the computer for most things. What is important is to provide a range of experiences. So being on the computer all the time is not beneficial.
When my grandson is with me, he falls asleep easily after having used the tablet. So do I, after reading a book on an electronic reader. The amount of time and when to allow screen time depends on what works for your family.
I suggest that it is better to not allow screen time in the morning when getting him off the screen is unpleasant. I would allow screen time after school for the amount of time seems reasonable to you. He'll still protest getting off the computer but he doesn't have to be anywhere else so can be ignored. Both my daughter and myself tell children to go to their room until they can be pleasant. Or take away screen time until they are able to get off the computer without a fight.
Just as many of us use ways of parenting that work for us, so is using the computer based on what works for us. I believe the idea of only allowing our children to use the computer for a limited time such as 30 minutes to an hour is not necessary for some children and some families. I use what works for my grandchildren and me.as long as I can see that using the computer as helpful.