You can file at any time after the discovery, but the time limit is based on several factors. Please see the IRS
Innocent Spouse Q & A. An excerpt from this is below;
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Balance Due – Generally, you must file your request within the time period the IRS has to collect the tax. Generally, the IRS has 10 years from the date the tax liability was assessed to collect the tax. In certain cases, the 10-year period is suspended.
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Credit or Refund – Generally, you must file your request within 3 years after the date the original return was filed or within 2 years after the date the tax was paid, whichever is later. But you may have more time to file if you live in a federally declared disaster area or you are physically or mentally unable to manage your financial affairs. See Pub. 556, for details.
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Both a Balance Due and a Credit or Refund – If you are seeking a refund of amounts you paid and relief from a balance due over and above what you have paid, the time period for credit or refund will apply to any payments you have made, and the time period for collection of a balance due amount will apply to any unpaid liability.