Probably 1.25", given that a normal dress belt is 1 inch. So with something like chinos, I could see going to 1.25". Whereas a 1.5" wide belt would look odd to me.
Maybe 1.5" would work with jeans. I don't know. I like to believe that even for casual Friday, jeans are too casual. I suppose that's not true at many places of work, but leave me to my fantasy, okay?