I might be able to answer this but not necessarily within the context you are thinking about. If one is to consider the nonlinear susceptibility of a model system, it would he helpful to determine what that response is regardless of the size of the system. If you look at the perturbation theory expression for the hyperpolarizability $\beta$, for example, it goes as $x^3/E^2$ and if you look at the Schrodinger equation carefully, the energy scales as $1/x^2$. So $\beta$ will get larger as the system gets larger regardless of any more fundamental change to the system. So, if you express your susceptibility in dimensionless form, that quantity means that there is something intrinsic to how it distributes oscillator strength among its excited states, how its energy spectrum is arranged, etc. You might say these are the more interesting features from a fundamental understanding point of view. 

We tend to use the off-resonant limit to the hyperpolarizabilities for our scale factor and I don't believe I've seen any reference to the electric field from a single photon in that prescription, so you must be referring to a different scale than what I am accustomed to.

http://www.sciencedirect.com/science/journal/03701573/529