Ultimately? With the death of the Sun.
Practically? When overpopulation and exploitation of resources renders it uninhabitable.
t.s.eliot was probably right: It won't end with a bang but with a whimper.
When will the world really come to an end?
Jesus prophesied: “Concerning that day and hour nobody knows, neither the angels of the heavens nor the Son, but only the Father. For just as the days of Noah were, so the presence of the Son of man will be. For as they were in those days before the flood, eating and drinking, men marrying and women being given in marriage, until the day that Noah entered into the ark; and they took no note until the flood came and swept them all away, so the presence of the Son of man will be.” (Matthew 24:36-39) That is why Jesus exhorts us to ‘pay attention to ourselves, and to keep awake, (spiritual) all the time making supplication that we may succeed in escaping all these things that are destined to occur.’ (Luke 21:34-36)
It doesn't end as you would think of it. Nothing ever ends it just transforms into something else.
Nobody, except for God knows. Not even Jesus knows. And it tells us this all throughout scripture. However, Scripture does give details that you need to unfog about how long the tribulation will last.
Just look in your bible in Revelations. Or search on google, KJV end of the world, and it will give you a whole bunch of scriptures to choose from!