When it comes to fondant, my belief is the thinner the better. But you have to work with a thickness that you are comfortable with. I use my semi-homemade white chocolate fondant and do it about 1/8" or even a little less.... but the chocolate fondant is stronger than straight from the bucket stuff. I would start at 1/4" and see if you can handle it. Go thinner if you can.
Jennifer 
I agree with Jen that the thinner the better, but also agree that you need to be comfortable with it. One of the big reasons I stayed away from fondant for so long is that I would see cakes with thick layers of fondant and I thought they looked like Play-Doh; I just refused to put my name on something that looked like a play-doh cake. Once I tried it and found it could be rolled thin and looked nice, then I was hooked!
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