@tehstepford
You sometimes join ongoing servers, or merge into on going server. I think the base number of hours of a server trying to shut down and merge everyone out is about 6 to 8 hours roughly, at least thats what I think. I've noticed is about close to 8 if not 8 when the server stops loading ships into the server.
I can't say I've ever "tested" it, but I after a hunting session, I'll notice less and less ships pop into the server as time goes on, after a while I notice that the only ship on the server other than me is the ship I'm chasing.
One other thing, I don't know if this is true or not. A player once suggested that the servers will sometimes merge even if there are 2 ships on the server, just really far apart. I think this might have happened to me. I snuck up on a brig and I ended up losing that fight, they were really on point with their cannons and shot me off. I know they had loot on their ship because I saw the shiny glow of the skulls on deck and mermaid gems when I got close. This happen at smugglers bay around there, when I grabbed mermaid to get my ship back, I spawned at Ancient Spire, the second I touched got on my boat, I server merged.
Now the brig crew could have just decided to not sell and leave. Who knows, but I don't think they just left without selling. That night was an 9 hour sess.