IRS rule is audits (and amending returns) can occur within 3 years of the date of filing a tax return. Then you and IRS are clear from revision, except….

IRS can and will pull returns from years previous to that – not necessarily to audit previous years, but for reference purposes (their records would likely be available to you too, should that occur). Meaning, they won’t necessarily audit those previous returns, but could use them to validate history if you “claimed” positions on those previous returns. So, you can get “stuck” with positions made from returns prior to that 3-year rule and there are times when records pertaining to older returns can come in handy, especially to the degree you are uncertain of positions taken or of the accuracy of your return.

Also, if IRS states on their website that should they find a “substantial error” or suspects fraud, they can audit backwards (generally up to 6 years from filing). Interestingly… as we Americans recently experienced during the Trump Russia Investigation, Paul Manafort was found guilty (not of anything Russia) but of IRS violations which occurred up to 8+ years prior to the time he was involved with Trump Campaign. So, in practicality, I don’t believe they have a “hard time frame.” With all that said, I don’t think you need to keep things past 3 years from date of filing – especially if you are a clean / honest tax filer.

Simple answer. Keep records for at least 3 years from date of filing – longer to the degree you are not comfortable with the accuracy of your filings. 🙂