It will probably take you 24 months. The schools offer nursing courses on a very regimented schedule. A cohort moves through the same nursing courses at the same time. There are no "optional" summer courses to take to speed you up. Either the school requires all to take a summer nursing course, or summer is time off for the entire cohort. Look at the college catalogs for the schools you are interested in. That will tell you how what you take and when. Nursing is not like regular college degrees, which you can stop/start, and move things around. It's very regimented.
In Pennsylvania, we still have diploma nursing schools. They require the same non-nursing college classes that associate degree RN degrees do. You must complete all college courses prior to starting the nursing courses, and then the time to do the RN part is anywhere from 16 months to 18 or 24, depending on the school. The 16-18 month programs usually run year 'round with only a few breaks. The 24 month ones may have summer off.