Rest: How long is long enough?

[SlamminP]

How long is the optimal time to rest in between work outs of the same muscle group? I've read and heard many different opinions about this. Some say a week is enough, some say 48 hours... this is a huge difference in opinion. I want facts.

Obviously you should never work out when your muscles are still sore, but the only time my muscles are really sore is if i haven't worked out for a month and then hit the gym. Once my body gets used to lifting, I no longer experience soreness on the same level (if at all). So how do I know when my muscles are fully prepared for another heavy weight raping?