A memory score on its own is a snapshot. It compares you to other people of your age and education, which is useful, but the range of normal is wide. Two people can both score comfortably within this span and be on completely different trajectories.
What matters is the direction your memory is heading, and you cannot see direction from a single point in time.
Population averages provide context. Your unique baseline shows how your results change over time.
For most people, once a year is sensible. It is frequent enough to catch a trend.
For the most meaningful comparison, take the exam under similar conditions each time. Choose a consistent time of day and a quiet setting, and avoid unusually stressful or exhausting days. Sleep, stress, illness and certain medications can all affect performance.
If something changes noticeably in between, do not wait for your annual anniversary.