In terms of time, DD1 is aiming for five hours a day and actually managing about three to four hours max.
She has the same issue with having some very time-consuming homework on subjects that she is already doing well in. Her English teacher has told them to type up the hand-written essays they did in the last mock and improve them as they go. But her typing is really, really slow, the essay is looooong and she already got a grade 8 in the exam, so it does seem pointless for her.
By the time she's done that kind of thing, the motivation for self-directed revision on subjects she really wants to focus on has gone. (See my rant above about the lack of science revision to date).
They have also been told that they need to clock up five revision interventions, in school, per subject in order to get their "passport to the prom". DD1 is not finding the school-based revision sessions helpful because they tend to cover stuff she is already doing OK in. I haven't decided whether to start a dialogue with the school. To be honest, given her lack of self-motivation to work at home, her argument that that would be better is wearing thin.