You will have to contend with a single Endbringer right as you arrive, which one is random. You might have an hour or so of grace since the Endbringer will need some time to get to your location. Use your preparation time wisely.
If the Endbringers do not exist in the world you are in, an attack will happen within a week of your arrival instead.
(If you picked Corona Pollentia, don't. You will die a horrible horrible death)