It's governed by diffusion. There is an equation ( Einstein eq. I think) that says something about the average distance diffused being proportional to sqrt(time

It's governed by diffusion. There is an equation ( Einstein eq. I think) that says something about the average distance diffused being proportional to sqrt(time) and a diffusion coefficient which depends on what is diffusing and in what medium. the units of the diffusion coefficient are in m^2/sec because of the square root dependence but it's sort of an analog to the speed. Delta x = Sqrt(6Dt) in 3d It's based on brownian motion leading to a gaussian distribution of distances travelled