I've done some Tai Chi and from what I've learned about it is that its not like the "hard" styles where you train in fast explosive moves but its a soft style where you go slower and smoother. So Tai Chi comes across to me as the kind of art you can do all day, 8 hours a day without burning out. To constantly train in a hard style 8 hours every day, a style such as Karate or Judo or Muai Thai, you will burn yourself out and your excessive training will become detrimental and you could hurt yourself but with Tai Chi perhaps not so. I imagine you could to Tai Chi 8 hours a day on a regular basis and not burn out.