There seems to be some misinformation in this thread.
There is a guranteed rotation of in world “cloud” events. Three forts spawn first and after the third is cleared or left alone for too long a fleet becomes active. Also I don’t necessarily deny any of your claims that they despawn too quickly (I have burned as well) however they definitely do not despawn in 10 or 15 minutes.
When the 3rd fort goes down (and by goes down that means the captain is killed and the cloud clears) the fleets will spawn in 2-5 minutes. That coincides typically with how fast you are off loading loot from the fort vault. The if you factor in sailing to an outpost and turning in and then sailing to the fleet there is almost no time for them to remain up.
Doing the forts back to back or at least one of them is still the best way to prepare yourself for the fleet. It allows you to control when the fleet spawns as well as gathering you the respurces and gunpowder needed to clear the ships.
In my experience they despawn in 25-30 minutes and I’ve timed it.
Also someone up above said there are inactive forts in the map and that is not true. Before today the only non active fort was the new one in the Devil’s Roar. All forts have been active since around shrouded spoils.
Lastly if you join a server and a fleet is up just don’t even bother. You won’t have enough time to gather adequate supplies. Just run the forts and if you bang those out you should be ready for the fleet to spawn in approximately an hour and halfs time.