72 swinger v8. Torsion bar question

They don’t get weak, the spring rate does not change.

They do sag, so eventually what happens is you run out of adjustment on the torsion bar adjusters and the ride height gets lower. As long as you can keep the ride height you need you can run those bars, assuming there’s no damage.

That’s said, if they’re original those are really old springs to be trusting your suspension to. And if you’re running radial tires, the factory spring rate is inadequate even if the bars are perfect.