I think a minimum of 2 days rest is good. I believe Cathe recommends 3-4 days of cardio to maintain, 4-5 days if you're trying to lose weight. (I could be wrong about this--for some reason it sticks in my mind but I'm not sure what my source is)
I do cardio 4 days a week, maybe 5 if I'm feeling really energetic, & it's a variety. Monday & Thursday I either run or do a step tape. Saturday & Sunday I do cardio at the gym.
I will say this however: in my experience the most effective cardio w/o is running. I know if I want to slim down fast I can run for all my cardio workouts for 2-3 weeks & have quick results. The downside is it's probably toughest on the knees.