Applying the strong pinning formalism to the mixed state of a type II superconductor, we study the effect of thermal fluctuations (or creep) on the penetration of an ac magnetic field as quantified by the so-called Campbell length. Within strong pinning theory, vortices get pinned by individual defects and jumps in the pinning energy (∆epin) and force (∆fpin) between bistable pinned- and free-states quantify the pinning process. We find that the evolution of the Campbell length λC(t) as a function of time t is the result of two competing effects, the change in the force jumps ∆fpin(t) and a change in the trapping area Strap(t) of vortices; the latter describes the area around the defect where a nearby vortex gets trapped. Contrary to naive expectation, we find that during the decay of the critical state in a zero-field cooled (ZFC) experiment, the Campbell length λC(t) is nonmonotonic, first decreasing with time t and then increasing for long waiting times, at least for very strong pinning. While the persistent current vanishes on approaching the equilibrium state at long waiting times t, e.g., above the irreversibility line, the relative change of the Campbell length during relaxation remains small. Finally, measuring the Campbell length λC(t) for different states, zero-field cooled, field cooled, and relaxed, as a function of different waiting times and temperatures, allows to spectroscopyse the pinning potential of the defects.