At least three years, but six years is recommended. The statute of limitation gives the IRS three years after you file a tax return to audit you. However, the IRS can audit you for up to six years if it suspects that you underreported your income by 25% or more. If the IRS suspects fraud, there is no time limit for an audit. Records of purchases of real estate, stocks, and other investments that prove basis should be kept for at least three years after the tax return reporting their sale was filed.