You can be audited the later date of either three years after the filing deadline of your tax return or when you actually filed your tax return. However, there are two main exceptions to this rule that can extend the risk of being audited;
If the IRS audits a tax return and discovers an error of more than 25% of your claimed tax obligation they can go back six years.
If the IRS deems there is fraud involved, they can go back indefinitely.
Every year the IRS publishes their examination statistics. Provided here are three years of published information to help you identify trends: