32 C.F.R. § 37.655
If your expenditure-based TIA provides for periodic audits of a for-profit participant by an IPA, you must specify the frequency for those audits. You should consider having an audit performed during the first year of the award, when the participant has its IPA do its next financial statement audit, unless the participant already had a systems audit due to other Federal awards within the past two years. The frequency thereafter may vary depending upon the dollars the participant is expending annually under the award, but it is not unreasonable to require an updated audit every two to three years to reverify that the participant's systems are reliable (the audit then would cover the two or three-year period between audits). The DCAA is a source of advice on audit frequencies if your TIA provides for audits by IPAs.
32 C.F.R. §37.655