There are many difficulties to recording an audiobook that are a challenge for even the best of narrators. From editing out mistakes, to ensuring that strange and distracting sounds are minimal to non-existent. As a rule-of-thumb, a typical narration is about 2.2 words per second. This is 132 words per minute.
*Please also note: The production is more than the voice over, but it is about 2-3 (or more) hours of work for a full edit and production.