When will English become the official language of the US?

For what purpose? It is already the predominant language, and you will never run into any obstacles conducting all of your business with the US government in English. What would be the benefit of officially having English as a national language?

It already is the official language of the majority of states, at the state level.  As for the federal level, well maybe Trump can get it through after he wins reelection & the House reverts back to a Republican majority.

All an official language does is require that government documents be offered in at least the official language(s).  Other languages can still be offered if desired.  It does absolutely nothing regarding the languages people use in business, public, or their personal lives.

Many states & territories have at least English as official (and some also list another language or more as official as well, such as Hawaiian and English in Hawaii or Spanish & English in Puerto Rico). 

Many states, like mine (Michigan) have no official language, just like the Federal government.

All state and federal documents are offered in English anyway, so what would be the point of making English official at the federal level?

It never has been; it never will be.

You'd have to get a lot of legislators to agree on it, and that won't happen.

We don't need an official language.  English is de facto official; it doesn't need to be de jure official, too.

de factual official - vs- de jury official.

Hopefully never as there is no need to do so. 
Were this to happen, people who only speak, read, write English would become
even lazier & more intolerant of those including myself who speak it and other(s).
Ironically we often speak, read, write it better than monolinguals, since we know 
what it's like to learn from scratch rather than expect to be able to, just because ..

Manana......heh.

We don't need an Official Language. We should just keep on speaking English, and schools should be in English (for the most part) and also teach Spanish, French and Esperanto.
Children for whom English is not a first language should have their schooling in multiple languages.