How long do I need to keep my tax returns?

This answer depends, but at a minimum, you should retain your tax returns for at least three years.  

Why three years? The IRS has a limit of three years to review your tax return and bill you for additional tax.  In addition, if you are seeking a refund, you have three years to file an amended return.  Now, if you omit 25% of more of your income in any year, the IRS can go back up to six years.

Once the three year mark occurs, do not throw-out everything out.  If you own real estate, you need those records to establish basis.  If you do decide to sell the property, keep the records for 3 years after you sell.