DS is prioritising the content-heavy subjects and those where he is weakest. So Science & Maths is getting less time than say Geography, English Lit and RS.
Aside from Tassomai, his Science revision is doing past papers, then revising any topics that he drops marks on. But we also look at grade boundaries for that paper and if he's scoring highly. They don't need to know absolutely everything inside out as long as they know enough for the grade they're targeting.
Same approach with Maths. Past papers, then looking over anything he didn't get and doing more questions on it.
The 'wordy' subjects are less straight-forward because they're much harder to mark even with the mark schemes. Most of his teachers have been happy to mark any practice questions he's done though.
He had a revision timetable over the Easter hols but hasn't had one since he's been back at school. He's just been revising stuff he felt needed it (for around 3 hours after school and 6 hours on weekend days). We're going to do a new one from Friday when study leave starts, tailored to when the exams fall.
Right now he's doing a Biology past paper then will spend the rest of the day and all of Tuesday on History for the exam on Wednesday.
I do think it can be counter-productive to think that you have to know absolutely everything in great detail for the whole syllabus. With the odd exception (RS and from memory Latin when DD did it) the grade boundaries allow for the odd topic that is understood less well and where marks will be dropped. DD (who is an arts and humanities girl through and through) got A*s in all 3 sciences and she definitely didn't know everything about everything. But she knew enough to get enough marks IYSWIM