Reservations include 180 free miles per day. So if you reserve for an hour, you get 180 miles. If you reserve for a day, you get 180 miles. Mileage above and beyond the 180 free miles is charged at $0.45 per mile for most cars ($0.55 for premium vehicles like the BMW).
For reservations that exceed 24 hours, you get 20 miles for each additional hour up to a maximum of 180 miles per day. So a 26-hour reservation would include 220 miles (180 for the first 24 hours + 40 for the additional 2 hours). A two-day reservation would include 360 miles (2 days x 180 miles). Geddit?
Mileage included varies by country: US reservations include 180 miles; Canadian reservations include 200 kilometers; and UK reservations include 60 miles.