Generally, the statute of limitations for the IRS to assess taxes on a taxpayer expires three (3) years from the due date of the return or the date on which it was filed, whichever is later. A return is considered to be filed on the due date of the return if it was filed on or before its due date. That would be the bare minimum time that you should retain copies of the tax return and all supporting documents.

I advise my clients to retain their tax returns longer if there is a specific reason. For example, if property is still owned that was acquired in a tax-free exchange, I would retain the tax return and supporting documents from the years of the original exchange until the property is sold or otherwise disposed of.