The maths
Minutes each way × 2, plus any other daily listening, × days a week × weeks a year gives your listening hours. Multiply by your speed for the hours of book you hear, then divide by the book length. Weeks default to 46, leaving out about six weeks of holidays.
Planning beyond the commute? The reading goal planner works from minutes a day, and the audiobook player plays your own books with a sleep timer and bookmarks.