They're like every other piece of steel. Sooner or later, they'll wear out.
This could take a zillion miles. Unlike coils, torsion bars tend to have adjustable mountings that can accommodate some yielding of the steel shaft.
Beyond yielding, they could fail from corrosion or work-hardening. Can't say that either is a common issue.
I bet most torsion bars get scrapped due to bending in a collision.