Over the years I have talked to several attorneys about this question, both inside the company and outside. Each has said to keep calcs, stamped/signed drawings, and other relevant data and files at least until the applicable statute of repose or statute of limitations (if any) has passed. In addition, professional liability carriers may have requirements for document retention. I have seen other posts here on Eng-Tips in which the advice from an attorney is reportedly to destroy drawings, calcs, etc as soon as the job is over, but I think that is terrible advice. I think it is far better to go into court or a settlement conference with, say, a less-than-perfect set of calcs that were at least done to the normal standard of care than to show up with nothing at all.
In structural engineering, where many projects are "one and done," I can at least see the temptation to clear out old files. In my type of civil engineering, where we may consult for a public (or even private) client for years, document retention is vital because the old files are part of the research we do for new projects. I have found relevant data in old job files that were decades old.
Project owners and contractors may keep copies of drawings and any calcs that were sent their way for a period of time after completion (the owner should keep at least the drawings permanently, though many fail to do so). Stamped and signed drawings are also a public record and the reviewing agency may have kept copies in their files; same with stamped and signed calculations that were reviewed by the agency. Some agencies around here will keep these documents for years, while others do not. I once found, in a local city's archives, a 10-year-old set of site civil plans that I needed for information for a new project at an industrial site. The owner of the site had lost his set a couple years before. Thus, destroying plans and calcs can pointless.
BTW, here are NSPE's guidelines for document retention:
==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill