M.H.
I have a son who has just finished first grade and a daughter who just finished preK. My son has to read for 15 minutes a day and it can be to his sister, to me or just by himself. He also has to do some math flashcards and practice writing. All told it is about 30 minutes a day. My daughter has to practice her letter sounds (through computer, being read to or watching a leapfrog video), practice her writing and her numbers every day....around 20 minutes a day. I've even got my 3 year old practicing letters. It isn't regimented or anything and they have say so over what they are doing as long as they are doing it....and they have a progress chart that they will get rewarded for at the end of the summer as well. Being a teacher myself I can tell you kids fall behind in the summer and I think it is good for them to have a little practice throughout. Be flexible and let them have fun with it but still do it! Find a fun book series for him to get into....mine loves Magic Treehouse and he loves it when I read Harry Potter to him as a break.
Good luck!