Home ed parents often recommend, especially for a child who has had a hard time at school, that you start off with a complete break from all formal learning. This gives the child time to recover his confidence and let go of the stress. It also gives parents time to explore some possible approaches to HE, make contacts with the local home ed community, and get used to being with the children for longer stretches.
In view of your older child's history, I would strongly encourage you to start off very gently and informally with him. Let him do all the things which make him happy, whether academic or not. Go swimming, build with Lego, read his favourite books aloud to him. Try to get him out of the house every day for a change of scene. His mental health is more important than anything, and he can always catch up on academics later. I would hazard a guess that he would benefit from having this entire year off from school to relax in a safe loving environment before possibly starting secondary school next year.
You might continue with this low-key approach in the long run; some parents find it is the only option for their children, and others simply like it best. If and when you do feel your son is ready for more formal parent-directed learning, start with his favourite subjects to get his confidence up and start off on a high note. Don't be hesitate to try different curricula, and change as needed, to find what suits him best. You have huge scope.
Are you looking to get your 8yo back into school as soon as a suitable place comes up? If so, you might just want to keep him ticking over with the basics (maths, reading, writing) and spend the rest of your time exploring any other subjects as they take his interest.
There is no fixed amount of time you must spend per day. Most of the families I know whose children are similar age to yours spend a couple of hours per day on sit-down work. Home education is far more efficient than school because you can work at exactly the right level for the child at all times. Other families, like mine, do no formal work at all. (However, it's worth noting that I never had any intention of sending my children to school at any particular age and so it didn't matter at all whether they were "ahead of" or "behind" their peers in specific subjects at any moment in time. For us, education was a long game, so it really didn't matter whether the children learned to read at four or seven or thirteen.)
Of course, you also have to look at which approach is practical for your family overall and what feels right to you personally. Look around at what other people are doing - going along to a local home ed group could be good for this and other reasons - and try something. Most families need to do a good deal of tinkering before settling on a method which seems right. That's okay; there is no hurry.