Seven years is sufficient for any circumstance. For most situatiuons, the IRS suggests three years.
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.
