Bear in mind that the fundamental burden of proof is almost always on the taxpayer in the U.S. tax system.  Evidence of income or deductions, in the form of cancelled checks, invoices, bills, bank statements and the like must be maintained by the taxpayer until such time as the period of limitations for the tax return expires.  So when does the period of limitations expire?

For the Internal Revenue Service to assess tax to you, these are the basic rules:

3 years - To assess tax to you, the IRS generally has 3 years from the date you filed the return.  If a return was filed before the due date, it is treated as filed on the due date.  If a return is filed on extension, the extended date is the beginning of the 3-year period.

6 years - A different period applies if you fail to report income that is more than 25% of the gross income shown on the return, or if the income is attributable to foreign financial assets and is more than $5,000.  In those cases, the time to assess tax is 6 years from the date you filed the return.

No limit - Not filing a return is NOT an answer, as there is no period of limitations to assess tax when you file a fraudulent return or when you don't file a return.

If you have a claim for a refund, these rules apply:

The later of 3 years or 2 years after tax was paid - For filing a claim for credit or refund, the period to make the claim generally is 3 years from the date you filed the original return (or the due date for filing the return if you filed the return before that date) or 2 years from the date the tax was paid, whichever is later.

7 years - For filing a claim for an overpayment resulting from a bad debt deduction or a loss from worthless securities, the time to make the claim is 7 years from when the return was due.

If you are in business, you must keep the accounting records and the underlying documents that support the income and expenses.  A business should follow the above guidelines as far as the length of time to keep its records.  A further caveat is that businesses must keep all employment tax records for at least 4 years after the employment tax becomes due or is paid, whichever is later. 

Unfortunately, another factor that comes into play is the need to prove the basis of property that was sold.  If property was bought 20 years ago, you will need to be able to present proof of that purchase (such as a closing statement), if challenged. 

Our typical recommendation is that a taxpayer should keep a copy of every annual return, without limit.  Simply put, the burden is on the taxpayer in the U.S. tax system and, if the IRS claims you did not file a return for a given year, you will be able to submit a copy of the return as evidence.  Maintaining those archived copies is also not the burden of your tax preparer; the tax preparer must give you a copy of the returns that were prepared, but the preparer is not required to keep copies of client returns forever.  While most states follow the guidelines of the IRS, just be aware that states can also have their own unique requirements.