As an ex-teen myself (haha), I'm 22 now and I can tell you I spent too much time on videogames. I don't think the games themselves are bad, rather it's that opportunity cost you're giving up. So, I could have been studying, or working out, or pursuing a hobby instead of gaming.

I'm not sure what number gaming should be limited to specifically, I just think that one should be focused more on other things. Lacking those things makes you less competent, less interesting, less skilled. So definitely help control your child's gaming, but the emphasis should be on giving them something to do, rather than just taking their gaming away.