nostr:npub1jk9h2jsa8hjmtm9qlcca942473gnyhuynz5rmgve0dlu6hpeazxqc3lqz7 Public free relays only store roughly 5-7 days of directed messages in my development experience. Or even less time depending on relay owner's setting.

If the client you are using doesn't store data locally, you will lost your old messages. Blowater and Iris stores DMs locally. (not sure for mobile clients)

Does this answer your doubt?