Tracy Jennings, Works at Noodle and here to help!
According to the most up-to-date data released by the US Department NCES, which was released in 2011, the median time was 52 months, or 4.3 years. Students majoring in the health care and computer science-related fields tended to take longer (57 months), while those majoring in humanities, social sciences, or natural sciences graduated on the earlier side (45 months).
(I assume median values are more useful, considering the average would be disproportionately skewed by long tale finishers.)
If you're interested in who cares about four-year completion rates, check out Noodle's report on what parents want for their children's college education.