At my son’s school they are “supposed” to get about 2x2 hours a week per subject.
Some children spend less than an hour, some more. Some don’t do it at all. The teacher might give a detention, but usually not.
Once they get into yr9 and they’re on the GCSE course snd they’re expected to revise on top of this eg make sure what they’ve learnt that week they know. This increases as they move through yr11.
Homework per se largely depends on the teacher. Some are excellent. Really engaged and bring their subjects alive. The homework they set is good to do. Others are not. They’re either underwhelmed by their job, emotionally elsewhere, possibly mid-divorce, waiting for retirement or struggle to make the subject anything more than words on a page. Their homework is usually a chore to do.
More often than not, the homework isn’t marked. I was surprised by this. Secondary is very different to primary. Grammar is even more self governed. It’s easy to fall behind. Grammar’s are focused on results and league tables. The number they can get into Oxbridge.
if your son is just over the pass mark, I’d think long and hard. It’s easy to become disillusioned and very average. Bright at primary doesn’t necessarily mean bright at Grammar.
My son is in yr11. I would say he’s pretty middle of the road in Year “ranking.” There are many who will likely struggle to get the GCSEs grades at the grammar that they would have at the non selective. The teaching at the other school in our town seems to be better in the sense that all children who want to do well are nurtured and can.
At the grammar, if you’re struggling in a subject or falling behind the solution usually is a tutor.
Don't for a minute think a grammar is a ticket to 10 x 9*. I think I thought it might be. If you’re a 6, you’re a six.
I appreciate it doesn’t answer your question, but I wish I’d have known this when my son was yr6. I’d have made different choices.