14 CFR 61.217: Recent experience requirements.

The holder of a ground instructor certificate may not perform the duties of a ground instructor unless the person can show that one of the following occurred during the preceding 12 calendar months:

(a) Employment or activity as a ground instructor giving pilot, flight instructor, or ground instructor training;

(b) Employment or activity as a flight instructor giving pilot, flight instructor, or ground instructor ground or flight training;

(c) Completion of an approved flight instructor refresher course and receipt of a graduation certificate for that course; or

(d) An endorsement from an authorized instructor certifying that the person has demonstrated knowledge in the subject areas prescribed under § 61.213(a)(3) and (a)(4), as appropriate.