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