Bachelor's degrees are 4 years in length. Most students take 4 years to complete them although it is not uncommon to finish in as little as 3 years or as many as 5.
The above is a general answer, but statistics from the College Board tell us that the average time to earn a bachelor's degree is 4.7 years (2008). So the trend is toward 5 years. Disturbingly, only 57% of those who start a 4 year degree finish within six years - so the above data does not count those who don't finish their degree.