The IRS says:
Keep records for 3 years if situations (4), (5), and (6) below do not apply to you.
Keep records for 3 years from the date you filed your original return or 2 years from the date you paid the tax, whichever is later, if you file a claim for credit or refund after you file your return.
Keep records for 7 years if you file a claim for a loss from worthless securities or bad debt deduction.
Keep records for 6 years if you do not report income that you should report, and it is more than 25% of the gross income shown on your return.
Keep records indefinitely if you do not file a return.
Keep records indefinitely if you file a fraudulent return.
Keep employment tax records for at least 4 years after the date that the tax becomes due or is paid, whichever is later.
The rule of thumb my CPA told me is 7 years. I keep mine until I run out of space. Have them back to 2002 presently.
I keep titles, lien releases, deeds FOREVER or until the property is no longer mine. Even then, I have them all in digital format (PDF) backed up. --173.20.xxx.xxx