Einstein and Nernst figured out that thermal agitation accounts for diffusion Hille chap 10, fig 1. Therefore, diffusion in water is extremely slow over distances greater than 1 mm. For a real membrane, which is not uniform, and probably has specific channels and other transporters, permeability is simply defined empirically. Osmosis can be balanced by a physical pressure to create an equilibrium. Tonicity is the osmotic pressure measured using a real membrane, usually a cell plasma membrane or an epithelial sheet.
Tonicity thus varies depending upon the characteristics of the membrane.
In practical units, for a monovalent ion at room temperature, the equilibrium potential increases 58 mV for each factor of 10 concentration gradient . By definition, at equilibrium every ion that is capable of diffusing across the membrane has its equilibrium potential equal to the membrane potential. Thus, any ion that has a different equilibrium potential is out of equilibrium, either because it is actively transported, is impermeant, or has not had a chance to come into equilibrium after a change in the membrane potential.