Preparation time can't be estimated. My explanation later.
Actual production time, for me, I think I spent about 4-5 hours on a 35 minute chapter. I'm a Virgo, we're perfectionists. NO, my work is far from perfect. But, I try.
I might spend more time in the record process, itself, than some others.
My initial editing time, editing out my mistakes, is probably about average. After that, I'll listen to the entire file. Shorten any long pauses, and try to fix, as much as I can, any mouth noises, excessive heavy breaths, tongue clicks, etc. This can be very time consuming, but, PROBABLY NOT NECESSARY.
I pause, a lot, during the record process. I stop to drink. Dry mouth, and, too much mouth noise, if i don't. Get up, move around a bit, stretch. (By the way, every time you stop record, SAVE YOUR FILE! Recording software, like Audacity, can crash, and you lose everything.)
I may stop to re-read a few paragraphs to make sure I have the right "feel".
Now, a word about prep time.
If I'm going to narrate a chapter in a book, I'll try to read the entire book, first. Then, I'll reread each chapter, before I record.
I'm finding that the language, wording, phrasing, used in the books I've been involved with on LibriVox, is a lot different than today's common speech. I need to look up a lot of these words, to understand their meaning, as well as to have the correct pronunciation.
The first 3 sentences of this post answers the question. Maybe I should have stopped there?