I generally re-trim on every flight. It all depends on how you like to fly. But changes in wind speed and direction may be a reason to re-trim. Also, I generaly like to fly at full or nearly full throttle, so when cutting the throttle for landing will cause the nose to drop. Sometimes I will then add a few clicks of up trim to compensate. Another thing is that as you burn off fuel from your tank, the trim will change slightly. Some people like to keep adjusting the trim to maintain a more neutral balanced plane.