This depends of course on what you are using for gluten-free flour, but according to Khymos' Data (PDF warning) and corroborated by my own experience, 1.5% is about the maximum concentration of xanthan you would ever use, beyond which food starts to get really slimy.
The recommended amount for flour is going to depend on exactly what kind of "gluten free flour" you are using. For example, you'll see at least two recipes in the link above, one using equal parts soy and rice/potato/corn starch and the other using rice flour and corn starch in approximately a 3:1 ratio (with tapioca starch added for what I presume to be heat stability).
However, xanthan gum is pretty forgiving, so you can experiment a little with the quantities and not have to worry too much. Personally, I would recommend starting with 1% as a baseline (that's approximately 1/2 tsp per cup) as well as 0.5% guar gum (1/4 tsp per cup). Guar gum has viscosity synergy with xanthan gum and will give you much more stabilizing bang for your buck.
If that's not stable enough, add up to 50% more (keeping the xanthan:guar in a 2:1 ratio). If it's too sticky/slimy, lower the quantity. Most recipes I've seen use close to a 1% ratio so that should be the best starting point.