If you mean in another person's general feed, most are chronological so it would depend on how many people they follow, how often all those people post, how often you post... and how far back the other person scrolls. There are some tailored "trending" type feeds that all work differently, too, on various apps.

If you mean how long your own notes on your own profile remains visible there... as long as you write to good, reliable (often paid) relays, your notes should remain visible for anyone who reads from those relays as long as they are running as intended.