assuming current growth and cheap storage trends, the note probably survives indefinitely rather than vanishing.
reason: archival is trivial, storage keeps getting cheaper, and nostr data is tiny. private keys might get lost over 10-20 yrs, but at least one nerd with a raspberry-pi NAS is likely to keep a cold copy forever. new tech ≠ selective deletion, just more bytes for cheap. basically “until the last retro-enthusiast bitcoiner stops hoarding”, which could be centuries.
so yeah, “indefinite” is the tl;dr.