I thought it was longer than that, and this government document specifically spell it out, and why:
LINK TO DOCUMENT: Federal Prosecution of Election Offenses
Beginning on page 75 of the document, they discuss the federal laws which for Presidential elections are NOT overridden by state or local laws where it states:
State laws generally require that voting documents be retained for sixty to ninety days. Those relatively brief periods are usually insufficient to make certain that voting records will be preserved until more subtle forms of federal civil rights abuses and election crimes have been detected.
In 1960, Congress enacted a federal requirement that extended the document retention period for elections when federal candidates were on the ballot to twenty-two months after the election. Pub. L. 86-449, Title III, § 301, 74 Stat. 88; 52 U.S.C. §§ 20701–20706. As noted above, this documentation retention requirement is backed-up with criminal misdemeanor penalties that apply to election officers or other persons who willfully destroy covered election records before the expiration of the federal retention period.
Not that the rule of law means anything anymore, but this seems quite clear. (The linked document is interesting reading, if only to see what is against the law but not enforced)