How long will it be before antibiotics become useless?

Useless? Never.

The key thing to keep in mind is that antibiotic resistance usually carries a fitness cost. Generally speaking, resistant bacteria grow more slowly than susceptible bacteria. This is true for most classes of antibiotics:

And it is true for most pathogens:

Figures from The fitness costs of antibiotic resistance mutations

It’s also generally true that the more resistances a bug carries, the less fit it becomes. But weakly virulent resistant bacteria can acquire compensatory mutations over time and become significantly more fit[1] . The first MRSA strains reported in the 1960s were completely unable to compete with susceptible strains except in the presence of antibiotics, but modern MRSA strains are very vigorous indeed.

Even though resistance rates can rise rapidly, they tend to plateau well below 100%

From Communicating trends in resistance using a drug resistance index

And with vigorous antibiotic stewardship programs, resistance rates can drop dramatically

From The English MRSA Miracle

We are not going back to the Golden Age of Antibiotics, when every prescription worked every time. But we are not going back to the Dark Ages either. Instead, we are entering a new Age of Prudence, in which antibiotics, when used judiciously, work almost all of the time. That is not a terrible fate.

Footnotes
[1] Compensation of fitness costs and reversibility of antibiotic resistance mutations.