The IRS normally will include returns filed within the last three years in an audit. If they identify a substantial error, they may add additional years. They usually don't go back more than the last six years. The IRS tries to audit tax returns as soon as possible after they are filed. So for safety most people hold the past 5 years of returns. Do note the IRS GET TRANSCRIPT feature maintains much pertinent data to recreate most simple returns.
Hope I was helpful.