I do about 6-7 days a week. I just take a day off whenever it's convenient- or well, whenever it's not convenient to work out.
My typical schedule is 60 min cardio (switch it up and do two machines, 30 min each, different machines depending on my mood- sometimes run, jog, speed walk, elliptical, total body machine, recumbant bike, stairmaster- pretty much anything). Then 10 min of stretching and ab work and twice a week I spend 1 hour doing the weight circuit plus about 35 min of cardio (maybe 8-10 min warmup, 25 min after) and lots of stretching after. I also sometimes throw in a class like body sculpt or step in there somewhere too.
If I take a day off, I usually try to take a 3 mile walk w/ DBF (takes about an hour).
I read somewhere that you shouldn't take off more than a day from working out. I find that if I do, I'm so much less likely to keep my momentum.
I also try to make the day off NOT the day after I do my weight circuit- my muscles need the stretching the day after or they'll get tight.