What conditions make xanthan thicken?
The viscosity of a xanthan solution is virtually unaffected by temperatures from
freezing point to boiling point of pure water and it hydrates rapidly in cold water. You don't need to let it sit on its own and the temperature doesn't matter.

The viscosity of a xanthan solution lowers when whisking or stirring, a process known as shear thinning. When you stop whisking, it will return to its previous viscosity. This will not factor in to thickening. That being said, you can get a xanthan gum foam using an immersion blender.

Between pH 1 and 13, the viscosity of a xanthan solution is practically constant. Unless you're using it for a strange application, you're good.

I cannot find any information on sugar or fat or other solids and their interaction with xanthan gum. Anything that can be dissolved in water would theoretically affect the xanthan gum's effectiveness, so I would expect sugar to be an issue, but fat or solids are obviously not.

The only thing you don't address is the concentration of your xanthan gum solution. Modernist Cooking Made Easy suggests a by weight ratio of:


0.2% ratio for light thickening
0.7% for a thicker sauce
and up to 1.5% for a very thick sauce


Additionally, the thickening effects only hold up for a day or two. What are you trying to thicken, specifically?

References


http://www1.lsbu.ac.uk/water/hyxan.html
http://www.lucidcolloids.com/pdf/xanthan.pdf‎
http://www.modernistcookingmadeeasy.com/info/modernist-ingredients/more/xanthan-gum