It's hard to pinpoint exactly when the poem leaves the "normal" world, because the speaker's version of "normal" involves acting like he's on the drug opium. But by the fourth stanza it has become clear that he has joined the nightingale in a dark, lush fantasy world. His journey takes him close to the experience of death, but the spell is broken when the bird flies away unexpectedly. The entire poem is characterized by the speaker's "altered" mental state, which he claims is not due to alcohol or drugs, although he compares it to these things.
The fantasy of the poem begins in stanza four, when the speaker escapes to the forest on the wings of poetry.
Keats wants the reader to think that his experience is identical to a state of extreme drunkenness or intoxication – only not caused by any kind of drug.