Good question! Check out 12 USC 2908.
For purposes of this discussion, "small bank" means a bank with assets of $250 million or less at the time of its last CRA examination. For banks of that size that were rated Satisfactory at the last CRA exam, the next CRA exam may not begin any sooner than 48 months from the close of the last CRA examination, even if your asset size grew to more than $250 million in that time. For banks of that size that were rated Outstanding at the last CRA exam, the next CRA exam may not begin any sooner than 60 months from the close of the last CRA exam.
For banks of that size that were rated less than Satisfactory, the regulators have discretion on what length cycle to use for CRA. Same thing for banks with more than $250 million in assets.
Hope this helps. AR.