26 CFR §301.6532-2

The United States may not recover any erroneous refund by civil action under section 7405 unless such action is begun within 2 years after the making of such refund. However, if any part of the refund was induced by fraud or misrepresentation of a material fact, the action to recover the erroneous refund may be brought at any time within 5 years from the date the refund was made.