It all depends. Traditionally it takes 4 years to graduate with a BS right. Well it seams to me that it takes 5 years for a large sum of people to for various reasons. So if you were to apply after your 3rd year with having 120 credits finished by the time you were to matriculate, just some of them are the wrong credits for a BS, many schools, not all by any stretch of the imagination, actual take this into consideration and basically figure you are prepared enough because you have as many credits for a bS just not the exact correct classes for a certain major, as long as you have a bunch of upper level biology classes done, very important. So it is not very much harder.
Now if you are talking about applying after only going to college for 2 yrs hoping to get accepted with only the min. 90 credits(3 yrs) by matriculation, now that is fairly difficult.