Do not leave your Diablo 4 World Boss too quickly, but do not wait forever either
If "Diablo walks the Earth" does not appear within seconds, you can probably head back home.
If you are farming 
Annihilus in Diablo 4 Season 15, you have probably already figured out that you need to stay after World Bosses, just in case some guy named Diablo decides to show up. But how long are you supposed to wait for Uber Diablo? Not only do we have no information on how to make him show up more often, but we also do not really know if there is a timer after a World Boss dies.
Thankfully, it does not seem like you need to camp the World Boss arena for ages.
Uber Diablo Appears Almost Immediately
When the Uber Diablo ambush triggers, the game displays the "Diablo walks the Earth" message after the World Boss dies, followed shortly by the Echo of Uber Diablo appearing in the same arena. Based on testing, he usually spawns within a few seconds of the World Boss dying.
So if by the time you picked up your World Boss loot, nothing has happened yet, you probably are not missing a secret interaction, and he will just not show up.
We still do not know exactly what causes the Uber Diablo ambush to happen. He can appear after different World Bosses, but the encounter definitely does not appear after every kill. There are multiple community theories about what might influence the spawn, but none of them have been confirmed officially.
For now, the strategy is just to go kill the World Boss, stay for a few seconds, and repeat until you get your Annihilus. No need to wait around 20 minutes, nothing will happen.