The knowledge of the spatial power spectra of the main geomagnetic field and of its secular variation makes it possible to define typical timescales t n for each spherical harmonic degree n. Investigating both observations and numerical dynamos, we show that a one‐parameter law of the form t n = t SV/n is satisfied for the non‐dipole field, given the statistical way the observed t n are expected to fluctuate. Consequently, we determine the corresponding secular‐ variation timescale t SV from either instantaneous or time‐ averaged spectra, leading to a value of 415 ± 45 55 yr for recent satellite field models. In the broader context of geomagnetic data assimilation, t SV could provide a sensible and convenient means to rescale the time axis of dynamo simulations.