We recommend to keep a copy of your tax return for a minimum of five years. Optimally, you should keep them indefinitely. Below is a table of the period of limitations for a tax return to be amended or audited:

If you...	The period is...
Do not report income that is more than 25% of the gross income shown on your tax return.	6 years.
File a fraudulent return.	No limit.
Do not file a return.	No limit.
Owe additional tax and the above situations do not apply to you.	3 years.
Amend a return to claim a credit or refund.	2-3 years after the taxes were paid.
File a claim for a loss from worthless securities.	7 years.

For tax returns involving property — rental property, depreciation, etc. — keep your records until they are no longer needed to compute gains or losses on the property or its replacements.

As for non-tax purposes, check with your insurance company or creditors. They sometimes require much older records than the IRS would.