Some people are whole books. Some are one chapter. Both change the plot.

Length is a poor way to measure whether a person mattered. Some people are around for a summer, a team, one class, and they still hand you something you keep: a habit, a taste in music, permission to be a certain way. Expecting everyone to stay forever turns natural endings into betrayals.

Think of someone who was only in your life for a few months and name what they left with you. If they are reachable, tell them, briefly and without making it strange. If they are not, let it change how you treat whoever is around for this chapter.