How quickly the E.coli gains the resistance against antibiotics?

[Eagle9]

As well-known, the bacteria (particularly E.coli) can gain the resistance against antibiotics. In Wikipedia its mechanism is written:

It may take the form of a spontaneous or induced genetic mutation, or the acquisition of resistance genes from other bacterial species by horizontal gene transfer via conjugation, transduction, or transformation. Many antibiotic resistance genes reside on transmissible plasmids, facilitating their transfer.

I would like to know:
1) Are there any OTHER mechanisms for gaining the resistance? Can the bacteria gain resistance when it has got antibiotic, LB broth in medium and nothing more? In other words, we have got only one species of bacteria (E.coli in this case), the antibiotic and LB broth, but there are no other species of bacteria that can give/lend plasmids to E.coli. So, can E.coli gain resistance in this situation?
2) How quickly do the bacteria generally gain the resistance? Can it happen during several hours or more time is needed?