I'd say that I tend to use the terms interchangeably. But I definitely do not call it an adventure when we play for a year, and I definitely do not call it a campaign if we do a one-shot. So there's a threshold in there somewhere. But I know that I describe it as a short campaign when I propose them to do a few sessions. But you're right that the difference is maybe in the structure: multiple narrative arcs, a certain complexity, character arcs, time to see change in the world.
But again, when WotC releases a 100+ pages large hardcover that clocks at I don't know how many thousands of words, I have no idea why that's an adventure. An adventure is shorter in my mind. When I buy a small zine from the OSR with a map, a few factions and a few points of interest, that's an adventure. As we complete the dungeon and the players say they want to go over the next hill, it becomes a campaign?