In my experience, when I had a Swamp attack at my small forward Sea Base, I portaled out to my Main Base, and I haven't returned to the sea base.
That attack has been locked in place for in-game months.
It was supposed to be just a quick stopover to grab something for a corpse run, when the attack occurred.

I haven't seen an attack at my Main Base since that time.

From: https://valheim.fandom.com/wiki/Events

For an event to end, a player must stay in the red circle for a time. Leaving the red circle will put the event timer on pause until a player returns.

Also, yes confirming that items despawn in a few days if outside the area of a crafting table.

EDIT: Shortened the wiki quote.