The phenomenon of transient diffusion of boron in crystalline silicon is considered. The local equilibrium assumption from the law of mass action is not used in order to estimate the length of the transient regime. For an interstitial diffuser such as boron, the pair of diffusion equations, one each for the mobile and the immobile species, can be interpreted as a generalized Fick's Law in the total concentration with a position- and time-dependent diffusion coefficient whose concentration-weighted average, called the global diffusion coefficient, has an analytic solution for an appropriately defined time-lag. At 850°C for boron, the transient in the global diffusion coefficient is negligible, showing that the local equilibrium assumption is justified. An estimate is given for conditions under which local equilibrium may not be valid.