As a general rule, your tax return can't be audited after three years from it's original filing date. If you filed before the due date of April 15, the 3 years starts to run from April 15 of the year it was due.
There are some exceptions of course:
1. If you understate your income by 25% or more on the return the audit deadline is extended to 6 years. It is not a good idea to under report income though and hang on for six years.
2. If you file a fraudulent return, there is no time limit on the audit. Tax fraud is any conduct that was meant to deceive the IRS. The mistake has to be purposeful. The common belief is that the IRS doesn't audit returns after three years even if there is some fraud indication if the amount of the debt is not above certain levels. I don't suggest clients rely on these claims.
3. The time limit only starts to run when the taxpayer files the tax return. Unfiled tax returns are always open to being audited. If six years have gone by and no return has been filed by you or the IRS than you may be safe because the IRS typically is not interested in personal returns unfiled for more than six years both from a civil and a criminal standpoint.
Audit notices are typically sent out about 12 to 18 months after the filing of the tax return, and the IRM or internal revenue manual instructs that audits are to be completed within 28 months after the return has been filed. This 28 month rule of course is just an internal deadline.
Time limits are a good thing for the taxpayer. The IRS has it's own set of problems that cause delays and the older the return is the more anxious they are to put it away.