We document magnitude-order displacement gradient variations along single faults as a function of microstructural style, hardening or weakening behaviour during deformation, and the variability of the former two over time during fault evolution in porous sandstone. The observed variations imply that the power-law displacement–length (D–L) scaling relation, Dmax = γLn, changes through time and stages of fault evolution. Findings of this study combined with published D–L data suggest that faults in porous rocks follow a three-stage evolution in D–L space: (1) proto-fault evolution with n ~ 0.5; (2) onset of macroscopic discrete slip with n in the range of 2–5; (3) stable growth with n ~ 1.0. This implies that the power-law exponent n depends on the maturity of the fault population D and the range of fault sizes under consideration. Thus, the relationship between displacement and length is more complicated than previously thought.