0 -> 15 minutes, I'm in what I'd call "conscious reading" mode.
15 -> 45 minutes, I start to see the world in front of me, but I'm still consciously reading.
After 45 minutes, I completely forget I'm reading at all - the process of going from words to imagination is automatic. For the most part, I'm just seeing a movie unfold.
Of course, this varies by book and what my mental state is at the moment. Some books are very heavy on descriptions, others heavy on plot. Some books, I never reach that "watching a movie" mode.
For me it is instantaneous. I pick up a book, and from the very first sentence I'm in the book's world. Sometimes I get jinxed by thinking about 'these are just letters'. That's almost like remembering that you're consciously breathing or being aware of the size of your tongue.
But I can get rid of that within a minute of determined reading.
On a related note, if I'm sick and I manage to get into a book despite the initial discomfort I can ignore my pain/sickness completely.