In an expanding universe that is modeled by the FRW metric we assume that scale factor of the "present epoch" is unity which is equivalent to a zero redshift. Therefore, most observed galaxies with nonzero redshifts are in our past light cone. But it is unclear to me how much back in time or far away in physical distance can we go to assume that scale factor is unity. Alternatively, I need to know when back in time we can no longer assume that proper distance is equal to co-moving distance. What is the physical distance limit of "today universe"?