- often multiple hours in the studio because people make mistakes, they get tired, they want to re-do passages etc.
- during recording you want at the very least a sound technician, the voice talent, and a director/author/producer to listen in and give direction/corrections/adjustments on the reading
- after recording you need at least an editor, a director/producer to make sure the recording is cut correctly.
And then off to the presses (that is, converting RAW audio to whatever digital formats).
A designer may also be involved if the book requires a different cover.
Disclosure: I work at Storytel though not anywhere near (other than physically) audiobook recordings.