Part 107 recurrent training: when and how (it's free)
Your remote pilot certificate doesn't expire, but your right to act as remote pilot in command does. To fly under Part 107 you must have passed the initial knowledge test, or completed recurrent training, within the previous 24 calendar months (14 CFR §107.65).
How to do it
Recurrent training is a free online course from the FAA (through the FAA Safety Team site, course ALC-677). There's no in-person exam and no fee.
Also required for night flying
Night operations need the updated test or training completed after April 6, 2021, plus anti-collision lighting visible for 3 statute miles (§107.29).
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Sources
Rules change. Check the current regulations before you rely on any of this. Not affiliated with the FAA.