We have heard the phrase "since time immemorial" to refer to an origin of some practice or belief embedded so far back in the past that its validity cannot be questioned. Interestingly, this is not just a vague term, because it has been specifically defined—more than once, as it happens.
William Blackstone, an English jurist (1723-1780) used the phrase "Time whereof the Memory of Man runneth not to the contrary." In England in 1832, this definition was adopted into law. If there was no record or (honest) personal recollection to the contrary, then a law or practice would remain unchallenged.
"Time immemorial" existed prior to 1823 or Blackstone, however. In 1275, the first Statute of Westminster—a collection of 51 clauses—determined that the time of (let's call it "modern" for its time) memory began on 6 July 1189, the coronation of Richard I Lionhearted of England. Events since then could likely be attested to using the records and memory of people. Any practice that existed prior to that was decreed to be so far back that contesting it was not worthwhile.
So there you have it. If it existed more than 826 years ago, don't bother arguing its validity. (In English law, anyway.)