If you dig a little and do your own research as you choose not to take my word for it, practically every study done protein (muscle building) synthesis rate returns to normal within 48 hours.
But you're still sore? It doesn't matter. You muscle has finished 'growing' from the workout you did two days ago and TECHNICALLY it can be trained again. So should you? Yes and no.
If you killed your legs two days ago with max out heavy squats that put a lot of pressure on your joints, tendons and so on, training that body part again is fine, but not at max effort. THOSE parts of the body, especially if you still feel sore, are still recovering, not the muscles. Always keep in mind that you're doing more then working muscles when you workout. You're putting mass amounts of stress your ligaments, cell walls, nervous and endocrine system too.
The more you train the more your body will adjust. The first time I did a serious leg workout I was on crutches for 3 days. And I wish that was an exaggeration. I even missed 1 day of work because of it. It isn't that I don't train just that hard to this day, but to an extent my muscles have come to understand the process, recover faster and heal.
The goal is to get the muscles to grow without having huge time lapses between training. My suggestion: Train hard and heavy and either A) do max 70% same effort when training same muscle again 2 days later, or B) let more time lapse before you work the muscle, say maybe 3-4 days to let the muscle AND tendons, ligaments etc heal at your max efforts.
Train, but train smart.