29.9785421 grams to be exact.
This has been talked about many times. In fact, someone talked about this just recently. Do a search in the next page or two of this section, and you might find that thread and what was discussed.
However, the correct answer is: No one really knows, and you're better off just eating a shitload of protein, just to be on the safe side of getting what you need to get bigger and stronger.