For me, it's when the first string gets scratchy and I can no longer get a pure tone. I can live with dead basses (if I'm not performing, which I'm not these days), but I hate a scratchy first string.
And, dead basses can be resurrected to a certain extent by tuning them down a 5th, pulling at them, and tuning them back up.
Amateurs practice until we get it right; professionals practice until they no longer get it wrong.
1999 Dake Traphagen (spruce)
2008 Dake Traphagen (spruce, double top)
2010 David Pace (cedar, double top)
2019 Antonius Müller (cedar, double top)