I don't think it said in the article, but I'd think that all of the data/evidence might be good to retain until all of the offender's appeals and related legal proceedings are finished.

After that, I think there's some reasonable level of effort that we should expect the government to have to make to only destroy what's illegal, but practically that might mean that other data gets destroyed as well. Maybe the owner can request specific things be saved or something like that; but it's likely to be something that needs to be considered on a case-by-case basis.