I was referring to the thick felt washers that are installed at the ball end at the tailpiece. They DO last forever.They don‘t last forever but as long as the set of Spiros last.
And they are only intended to be used for the high(er) string until they had broken in to mellow the sound of a new string.
Therefor they are called tone filters.
I now see that you all are referring to the rubber washer that was used as a timbre/tone filter.
Sorry for my error.
Carry on...
Thx.