[gregdunn]

The 3 years/10^18 figures I used were taken from https://arxiv.org/pdf/1304.2821.pdf :

"Using the largest allowed value for the photon mass from other experiments, we find a lower limit of about 3 yr on the photon rest-frame lifetime. For photons in the visible spectrum, this corresponds to a lifetime around 10^18 yrs."

Which does work under the assumption that photons have a non-zero rest mass, and as such, could decay. I probably should have phrased my post better - this is hardly settled science, and the majority of physicists would almost certainly say that photons, with our current accepted theories, do not have rest mass. SR is pretty explicit on this :). But we can't say for sure that photons have zero rest mass - experiments have allowed us to set an upper bound on the rest mass, but not allow us to deterministically say they have zero rest mass. In such a case that photons do have rest mass, we would need to stop saying "The speed of light", and instead say "The speed of a (rest) massless particle", though this might be worth doing in general - gluons (and gravitons, if they exist) should be similarly massless.