The most I've done is five but this last vacation I took I was doing about four a day for ten days straight. And after all that time on the water I still suck.
I took four average lenght sets one day on the backwater at Canyon and the next day I thought I was going to die. Two pretty long sets a day is good for me.
30-45 minutes of riding. Riding could include riding the whole time, doing tricks and only falling here and there, but if I'm trying to learn something then I spend a lot of that time sitting in the water.