I sang 12 hrs straight with practically no break and do it a lot. Is this ok?

Voice coach. I agree with dr nguyen. But if you are singing this much it might be worthwhile to have an eval with a singing coach who is experienced or a speech pathologist. Often they can go over some techniques that can either help you maintain your voice or diagnose bad habits that will prevent you from abusing your vocal cords. Fyi world voice day is april 16th!
Career singer? Sing us a song.....Well, many peope sing for a living and so use their vocal cords quite a bit. I don't believe there is a known limit. As long as you don't abuse them or get hoarsenss often etc...And you feel ok, then i think you are ok. Just use your commonsense and you will be fine.. So when are you going-live on healthtap? :-) good luck..