When everyone knows you have worked for IRS, this question alone, bar none is the most frequently asked question alone with,  Why was my return pulled for audit? To answer the how far back can the IRS go, hope this answers your question. Let a former IRS Manager Represent you during a tax audit
How far back can the IRS go to audit my return?
Generally, the IRS can include returns filed within the last three years in an audit. Additional years can be added if a substantial error is identified. Generally, if a substantial error is identified, the IRS will not go back more than the last six years.
The IRS tries to audit tax returns as soon as possible after they are filed. Accordingly most audits will be of returns filed within the last two years.
If an audit is for an older year, you may be requested to extend the statute of limitations for assessment of your tax return. The statute of limitations limits the time allowed to assess additional tax.
The statute of limitations is generally three years after a return is due or was filed, whichever is later. There is also a statute of limitations for making refunds.
If the audit is not resolved and the statute of limitations date is nearing, you may be asked to extend the statute of limitations date. This will allow you additional time to provide further documentation to support your position, request an appeal if you do not agree with the audit results, or to claim a tax refund or credit. It also allows the IRS time to complete the audit and provides time to process the audit results.
You do not have to agree to extend the statute of limitations date. However, if you do not agree, the examiner will be forced to make a determination based upon the information they currently have. Therefore, the examiner may not be able to consider additional adjustments, such as expenses, that could lower the amount of tax due.
1% of all tax returns are audited.
1.4 million mail correspondence audits take place every year