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We recently presented a new statistical method to constrain the physics ofstar formation and feedback on the cloud scale by reconstructing the underlyingevolutionary timeline. However, by itself this new method only recovers therelative durations of different evolutionary phases. To enable observationalapplications, it therefore requires knowledge of an absolute 'referencetime-scale' to convert relative time-scales into absolute values. The logicalchoice for this reference time-scale is the duration over which the starformation rate (SFR) tracer is visible because it can be characterised usingstellar population synthesis (SPS) models. In this paper, we calibrate thisreference time-scale using synthetic emission maps of several SFR tracers,generated by combining the output from a hydrodynamical disc galaxy simulationwith the SPS model SLUG2. We apply our statistical method to obtainself-consistent measurements of each tracer's reference time-scale. Theseinclude H${\alpha}$ and 12 ultraviolet (UV) filters (from GALEX, Swift, andHST), which cover a wavelength range 150-350 nm. At solar metallicity, themeasured reference time-scales of H${\alpha}$ are ${4.32^{+0.09}_{-0.23}}$ Myrwith continuum subtraction, and 6-16 Myr without, where the time-scaleincreases with filter width. For the UV filters we find 17-33 Myr, nearlymonotonically increasing with wavelength. The characteristic time-scaledecreases towards higher metallicities, as well as to lower star formation ratesurface densities, owing to stellar initial mass function sampling effects. Weprovide fitting functions for the reference time-scale as a function ofmetallicity, filter width, or wavelength, to enable observational applicationsof our statistical method across a wide variety of galaxies.

An uncertainty principle for star formation -- III. The characteristic emission time-scales of star formation rate tracers