While I agree with dragonsmurf that there are safeguards in place to prevent potential issues with your hardware, both in the short and long term, I do try to take breaks from the game from time to time. Usually I game for an hour then give my rig 20 to 30 minutes to cool down. I have logged over 200 hours in the game thus far, yes I have no life outside of my job I don't want to hear about it, and my computer has yet to show any significant problem.
As long as you are not deliberately forcing your hardware to grind over its limits by such techniques as overclocking or overrides, it is very likely that you will be fine. Obiously if your computer starts shutting down durrign gameplay due to temperature overheat, its a good sign that you should hold off on the game until a patch comes out that optimizes the game. That or upgrade your rig.
Hope this helps.
