I have used some Kumon workbooks in the past, but mostly for writing. My opinion of them is good and bad. They do seem to be low-stress, and at their best can be effective for some things. On the other hand, some of the Kumon material I've seen has given the impression of placing too much emphasis on drills.
For math specifically, I have read lots of good things about the Singapore Math site and materials. I have read of lots of other good resources here (Life of Fred, etc.) that I have yet to check out, though I plan to do so. The main math extracurricular resource I currently use with DS5 is IXL. I tend to use it as a curriculum guide as well as a skill/confidence booster. Using IXL and my own homegrown approach to teaching math, my son has progressed pretty quickly so far.
I don't think any certain number of minutes per day is too much. The real questions for me would be whether your son is learning, whether he is enjoying himself, and whether he is in danger of burning out. If your son gets started on reading and reads for an hour because he doesn't want to stop, I'd be happy that I have that sort of happy reader for a son.
My son currently brings home homework that is far, far beneath his readiness levels in essentially all ways except for handwriting; he's actually embarrassed to talk about his homework, even with us. Like you, we have him do the work for now. We see value in learning to follow school rules, and his handwriting still does need some work.
