I probably would recommend no more than 3x a week of Retin-a, and no less than 1x a week. At first you'll peel a lot, and get red and irritated until your skin starts to adapt. Apply the Retin-a at night to keep your scalp out of the sun, and you're good to go. I apply the minoxidil thirty minutes after the Retin-a to maximize the application, and minimize irritation.
I just recently switched over to .025%, after running out of .05%. Probably anything above .025% is overkill for hair growth. I had even stopped using Retin-a for a little more than a month, and my great results remained. In my opinion, Retin-a really gives a super long-lasting effect compared to the fleeting effects from minoxidil alone.