What is the proper amount of rest required between working out major muscle groups?

I have been working out for a couple of months now. I eat right, supplement right(whey, casein, NO, creatine, 3,000-5,000 calories per day with lots of fruits and veggies too), and just want to make sure I do everything else right to get the most gains. I have gained 12 lbs so far, I would say half of that is muscle. Here is the question... Let's take my "back day" for example. I do a viscous back work out, but feel like I could do the same work out the next day with no ill effect. When I started, the next day I could feel that my muscles had been worked very hard, but lately I have not felt "soreness" at all. I never do the same work out twice, always mixing it up a bit to shock'em, and I never work out the same muscle group twice in a week. If my muscles are not sore, is it ok to work out the same muscles twice in a week? I use GNC products so I know that my recovery time is sped up, but don’t want to fatigue the muscles again before they are fully recovered which I know can be counterproductive. Thanks in advance.