The IRS announced that an estimated one million taxpayers who did not file an income tax return in 2013 could claim their share of $1 billion in unclaimed refunds for the 2013 tax year. The law gives most taxpayers a three-year time period to claim a tax refund. After that time, the money belongs to the U.S. Treasury. So if you did not file in 2013, to be safe, send your 2013 tax return via certified mail to arrive at the IRS by April 18.