15 CFR § 90.6 - When a challenge may be filed.

(a) A request for a challenge to a population estimate may be filed any time up to 90 days after the release of the estimate by the Census Bureau. Publication by the Census Bureau on its website (www.census.gov) shall constitute release. Documentation requesting a challenge of any estimate may also be filed any time up to 90 days after the date the Census Bureau, on its own initiative, revises that estimate.

(b) If, however, a governmental unit has a sufficiently meritorious reason for not filing in a timely manner, the Census Bureau has the discretion to accept the late request.