For unfiled tax returns, criminal violations or fraud, though, the practical limit is usually six years. Another scary rule is that the IRS can audit forever if you omit certain tax forms. Plus, once an assessment is made, the IRS collection statute is typically 10 years. In some cases, the IRS can go back 30 years.
In addition, if you haven't filed your taxes from previous years, you must do so within the three years in order to get a refund. Past year returns must also be filed in paper form directly to the IRS. It takes 10 to 12 weeks to process an amended tax return and it could take longer to process past year returns.