That would be a tough thing to pin down. Judging by some of the things Isaiah prophesied before Judah's fall, the Lord "sealed up the vision," meaning the prophesy had been declared and now nothing would be allowed to turn it back. So He had this way of closing the ears of the people once judgment was set, lest they repent, and what had been prophesied be proven to be a lie.
In that context, I would say beginning not long after Amos' prophecy went forth. But that is admittedly conjecture on my part.
IN the reading of the whole of it , its saying the word , they wont hear a word FROM GOD . Its like GOD has said in other places ,
THEN shall they cry , but i will not answer them . Once the judgment begins . And notice
if we read a bit farther it says those who swear by the sin of samaria and say thy God o dan liveth .
WE can see this is a judgment against the ungodly . NOT against true believers . The true beleivers will always hear from God .
This has nothing to do with mandela effect at all . There are people who believe
that satan has , or science etc , opened a portal and they actually changed the bible . THIS IS NOT TRUE .
But we got folks believing it is . And amos does not back this theory up one bit , for this judgment AMOS spoke
of applied to the ungodly , NOT to the godly .