How much screen time should my 13 & 15 year old boys be getting during the week?

They could both be doing much better in school and my older son is going into year 10. They are currently cut off at 10pm and basically have unlimited screen time when they are not doing sports or training. I’m finding that on the rare occasions they have homework, they are not doing it on rushing it late at night. I don’t want to be too strict as I want them to be able to have some downtime with their friends online Monday to Thursday night I feel like I’m allowing them too much time at the moment. Has anyone else found a good balance? What are others doing that is working for them and their teenagers?