The assessment period is extended to six years instead of three years if you do not report income that you should report, and it is more than 25% of the gross income shown on your tax return.
http://www.taxformprocessing.com/faq/how-long-do-i-need-to-keep-tax-records.htm
There is no time frame for the IRS to access tax and tax records must be kept indefinitely if you do NOT file a tax return, OR you file a fraudulent tax return. You must keep records for 7 years if you file a claim for a loss from worthless securities or bad debt deduction.
http://www.taxformprocessing.com/faq/how-long-do-i-need-to-keep-tax-records.htm
How long you should hang on to records. IF you ... THEN the period is ... File a return and the next 3 situations below do not apply to you. 3 years. Do not report income that you should and it is more than 25% of the gross income shown on your return.
http://www.bankrate.com/finance/money-guides/how-long-should-you-keep-tax-records.aspx
View all. I am often asked how long one should keep tax records. Naturally, the answer is not cut and dried. For example, the IRS states that you must retain your tax returns and all supporting documents including bank statements for a minimum of three years, which is generally how long they have to audit a tax return. If you live in a state that levies an income tax, the rule is generally expanded to four years.
http://www.foxbusiness.com/features/2014/10/02/how-long-should-keep-my-tax-records.html
Certain documents should be saved longer. Information related to your home, property, investments and retirement plans should be kept indefinitely, says Dolmage. If you dispose of an asset, be sure to keep the information for another three years.. Business owners should keep tax information for at least four years. That includes employment records, gross receipts, invoices, bank statements, proofs of purchase, asset records, databases, emails and even voicemails.
https://www.taxact.com/tax-information/articles/your-tax-documents-what-to-keep-and-shred
Certain documents need to be kept permanently while others should be kept until the statute of limitations expires. The statute of limitations for federal tax returns is usually three years from the date the return was due or filed, or two years from the date the tax was paid, whichever is later.
http://www.taxformprocessing.com/faq/how-long-do-i-need-to-keep-tax-records.htm
What to keep after filing your taxes and how long. Knowing what information to save and for how long can be confusing. As a general rule of thumb, keep tax returns and related documents for at least three years from the April 15 filing deadline.
https://www.taxact.com/tax-information/articles/your-tax-documents-what-to-keep-and-shred
Keep for seven years. If you fail to report more than 25 percent of your gross income on your tax returns, the government has six years to collect the tax or start legal proceedings. So when it comes to determining how long to keep tax records—electronic and paper— we recommend seven years, just in case. Keep forever.
http://www.consumerreports.org/taxes/how-long-to-keep-tax-documents/
Here's how long you should keep these old tax records. Maintaining tax filing documentation is critical. It contains the answers to questions the Internal Revenue Service could ask years after you file your return.
http://www.bankrate.com/finance/money-guides/how-long-should-you-keep-tax-records.aspx
Why should I keep records? Good records will help you monitor the progress of your business, prepare your financial statements, identify sources of income, keep track of deductible expenses, keep track of your basis in property, prepare your tax returns, and support items reported on your tax returns.
https://www.irs.gov/businesses/small-businesses-self-employed/recordkeeping
how many years of tax records must i keep
There are 3 years of  tax records must i keep. You must keep tax records for 3 years.