BSN programs usually run 8 full time semesters (that's 4 years). 1 year for pre-reqs and then 3 years in the nursing school
ADN programs usually run 5-6 full time semesters. The ADN program I'm in only required 1 semester of pre-reqs, or, for the pre-reqs they required you could get them in 1 semester, and the program last 2 years. HOWEVER, that's sufice to say you can get all of your required credits in 5 semesters; it might take longer. I'm going into an ADN program with 2 years of college under my belt, so I could go part time from here on out and get by.
If you go for an RN-BSN, which is the way in which you achieve a BSN degree after you've already become licenced as an RN (majority here are ADN). That varies. For my RN-BSN I'll have to go to school another year and then take a couple summer classes.
There is tons of diversity among nursing schools
, so just research.