3 years. Under the Tax Code, the IRS must assess tax within 3 years after the return is filed. That includes assessments due to an audit. Beware, the Tax Code also allows the IRS to go back a total of 6 years if the taxpayer understates at least 25% of gross income in the return. Note, these periods never run if 1) the IRS filed for you, i.e. what the IRS calls an SFR (Substitute For Return); or 2) if the IRS deems the return false or fraudulent in any way. For these reasons, you should keep at least 7 years of returns on hand and ALWAYS file and report your income and deductions truthfully.