Begs the question why your neighbor asked given he doesn't understand every boat has some means of preventing water from entering the shaft hole (or rudder post hole).
As to when to adjust, that is a function of what type. The traditional stuffing box should be adjusted for a minimal drip while turning to ensure some water flow for cooling the shaft. It is an integral step in routine maintenance of every boat which I infer you are not doing. If you are unsure of what to do and how often, you would be well served to read up on the subject and how to repack the stuffing on a routine basis.
This is a fundamental step in the maintenance routine, or should be..