It should go back as long as the test stays below the yield point: https://en.wikipedia.org/wiki/Yield_(engineering) ... but there is also fatigue: https://en.wikipedia.org/wiki/Fatigue_(material) . This is just for raw material though, I'm not sure how it works on the connectors.

Those struts are also used in the stages that are about to be reusable, so my guess is that they're supposed to withstand the flight conditions multiple times. That would hint the equivalent tests should also not be destructive to the material.