The internal Revenue Service in most states has got 3 years in which to audit Federal income tax returns. This limit may not apply in unusual cases for instance if you failed to report more than 25 percentage of your gross income, the Government has 6 years to collect the tax or better still start legal proceedings. There are no specific time limitations if you filed a fraudulent return. Visit http://publications.usa.gov/epublications/keeprecords/keeprecords.htm for more information.
The government (IRS) says at least three years. If you commit or are accused of committing Fraud they can go back as far as they need within the statute of limitations. If one has cheated or is suspected of cheating by the IRS, the Service can go back at least least 6 years but may have to reconstruct income from records as many who cheat do not file returns. Some of the off-shore tax haven banks are starting to cooperate recently with the IRS and are revealing some of their American clients to the U.S. tax authorities so you may not be able to safely hide your income by sending it overseas in the future.
You are legally required to save documentation of anything used on the tax return for 3 years from the original due date or date of filing (if filed late). After that, the return is considered "closed" except for fraud.