It's a while ago now, but with our DS, by the time the exams (GCSE and A levels) were "in sight", i.e. several weeks away rather than months, we were completely hands-off and left him entirely to make his own decisions.
BUT, that was after months/years of preparation for exams, right back to the start of secondary school, where we were "helicopter parents" from day one, checking his homework diary, checking his exercise books each week to see what he'd been doing, see what marks he was getting for homework, etc., "making sure" he revised for regular tests such as spelling, MFL vocab, etc., making sure he had his own space to work, plenty of "materials" i.e. all the equipment required (Maths geometry sets, coloured pens, highlighters, sticky notes, etc), bought him supplementary text books, revision guides, etc. Also showed him online revision technique resources (yes right from year 7) such as how to write/use revision cards, sticking post it notes all over his bedroom, and various other techniques such as online quizlet, A4 templates for study/revision etc. Year 7 was really quite intense for us and him, but for us it was all about getting him into good habits (i.e. doing his homework and revising for tests), and learning how to study/revise effectively so he'd see the end results from his efforts.
Once we'd embedded those good habits, we stood back quite a bit over years 8 and 9 and trusted him to maintain it himself, which he mostly did, and we just monitored his work occasionally, i.e. not actually looking at his homework diary or exercise books, but we'd just casually ask how the test went, or his mark for his last major homework assignment, so "interested" rather than obsessed and controlling.
For his GCSE years, we were completely hands off as we didn't want to stress him, we trusted him to do the work, and anyway, a lot of what he was studying was beyond us anyway, so we'd not be able to help/understand most of what he was doing, so we'd be more of a hinderance than a help. We left it for him to tell us what resources he needed, i.e. GCP revision/exam practice books, Letts guides for Eng Lit, etc and bought them for him. We just made sure he had the time/space to study, i.e. tried to leave one day every weekend "free" from family events/visits/trips etc, likewise tried to leave 2/3 evenings clear, - again, no "pressure" for him to use that time to study, but it was "free" if he wanted/needed to. Likewise we kept the house "quiet", i.e. no loud music/TV or other disruption in the rest of the house during times when he may want to study. It was really all more about not putting barriers/obstacles in his way that may have hindered/prevented him from studying. By the time of the mocks at the prior Xmas and then the second mocks at Easter, we kept out of his way, didn't interfere and just "let him be" - some days he'd study, others he'd be out with friends etc., but we didn't intrude at all. He knew we were there if he needed anything and he'd tell us if he needed another revision book, or another pack of cards or whatever.
I think that when you get very close to the exams, it's counter-productive to be too "involved" as it just stresses them out. If they've not kept up with the curriculum during lessons, not done the revision/learning for progress tests in the early year(s), then cramming in the last few weeks really isn't going to make much difference, especially if they're doing 7/8 subjects each with maybe 2/3/4 different exams. It's a marathon and not a sprint. The last few weeks "revision" are all about reminding themselves of thing they should previously have learned and understood - it's not about learning new things.