Ds(15)'s homework handbook says S3-4 should do 1-1.5hrs a night of homework and/or self study, they give out very little homework
so they do need to make the effort to self study/revise. I made encouraged ds to do an hour most nights in S3 and then it was 1-1.5hr in S4 depending on what else he had on. Worked out roughly an hours study per subject. It has worked well so far especially for maths and science, he is in the habit of it now, sees the benefits in class tests and will hopefully take this study ethic through to higher.
Dh if left to his own devices would probably let him away with less, but knows better than to interfere or contradict, he is seeing the results in his marks and confidence and agrees it is working! (dh barely attended school in the 80s, left with no qualifications and agrees he is lazy when it comes to learning!). If your dh isn't supportive you could agree to start with 30-40mins per main subjects (English, maths, science if taking etc) per week to begin, with a view to increase in S4?
I had to spend many hours some time with ds in S3 researching how to revise effectively for each subject, it was a bit hit and miss to begin with but we got there and he studies independently now he knows how. Buy revision guides and really show an interest in what he was doing, get involved if needed, lots of encouragement /praise. Try to do that now in preparation for the ramp up of work in S4.
If he is struggling in a certain subject and budget allows think of a tutor (one of ds's friends did maths at kip mcgrath for a few months in S3 and said it helped) to get him back on track before he falls further behind, or some of the revision guides could help you tutor him (even if it is just to sit with and support him working it out himself/let him try to explain to you what he knows and help him research the gaps - ds and I painfully worked out quadratics together with a youtube video and it has stuck as he had to work it out!!)