I'd assume it depends a lot on how close the source is to the present. But what is it like for the various eras? Do we ever find new written material about societies like Rome or the early caliphates? Or are there too many sources and not enough historians to read them all?
When it comes to the Roman Empire, we find new papyri in Egypt all the time, though these tend to be rather fragmentary. You can read more about it in this thread by u/LegalAction and this one with answers from u/KiwiHellenist, u/MichaelJTaylorPhD, and others