I think it depends on what you mean by "working out". Do you do weights or cardio or both? I like to do cardio on MWF and do Pilates/Weights on TTh. Sometimes I'll do cardio on Sat or Sun, but on the weekends I like to do other non-gym type activities like biking, hiking or swimming. It just depends on your body and what it can handle - it definitely needs some rest! You need to change up the activities once in a while because your body will get used to doing the same thing over and over. Mostly I use the elliptical but once a week I'll run on the treadmill instead (which I HATE!). And sometimes I'll work out on my exercise ball instead of Pilates...
Some people like to lift weights 5-6 days in a row, but they alternate muscle groups (legs on M, arms/chest on T, legs on W, etc). It just depends on what you like, but I think the most important part is that you stay active! Listen to your body, it will tell you when it's had enough!