Income taxes are generally filed once a year, which usually results in either a refund or a payment to the IRS and/or state government, on taxes owed to you or by you respectively. IRS guidelines have suggested retaining tax returns and supporting documents for several years depending on the situation.
1) Federal and state returns
2) Receipts and statements relating to charitable contributions
3) Supporting documentation for earned income, investment income, and expenses
4) Records to document capital gains or losses, such as trade confirmations