You don't need to keep it at all if you didn't declare these expenses on your taxes. If you did report them on your taxes then I'd suggest keeping it for three years because that is the length of time the IRS can go back to audit you. Some suggest keeping it for six or seven years because the IRS can go back six years for an audit if you failed to report 25% or more of your income, but as long as you have been diligent about reporting things correctly you don't need to hold it that long.