About how old is the light we see from the Andromeda galaxy?

Answer

Andromeda is about 2.5 million light-years away, so we see it as it looked when early humans were appearing.

Context

Andromeda lies about 2.5 million light-years from us, and a light-year is the distance light travels in a year, so the light now reaching Earth left on a journey of roughly 2.5 million years. Looking at Andromeda is therefore a look into the past: the photons that reach a telescope tonight set out when early members of the human genus were living in Africa. Because light takes time to cross space, astronomers can never see any object as it is 'now'.

Distances to Andromeda were first pinned down by Edwin Hubble in the 1920s, who found Cepheid variable stars in it. These stars pulse at a rate linked to their true brightness, so comparing that brightness with how dim they appear gives a distance. His result showed that Andromeda was far outside the Milky Way, settling the debate over whether such 'nebulae' were separate galaxies. Modern estimates, using Cepheids, eclipsing binaries and other methods, are close to 2.5 million light-years with a small uncertainty.

Andromeda is the most distant object commonly visible to the naked eye from dark sites, appearing as a faint smudge about six times the width of the full Moon in the sky, though only its bright core is easy to see. It is also approaching us, so in a few billion years it will merge with the Milky Way.