For most ADN to BSN programs there are no "pre-reqs". The "pre-req" is the ADN you have.
Some have co-req courses, but that varies from college to college. For example my program required a college level chemistry, statistics, two religion courses, and a humanities (Western Civ.) that I didn't have. I didn't have to have them to start the ADN to BSN program, but I do have to have them to graduate, so they aren't "pre-reqs" but co-reqs.
might require two years of forieng language, no chemistry, etc. It just depends. Almost all require Statistics as a co-req.
I took my co-reqs along side the ADN to BSN courses, one by one, accellerated to 7.5 weeks per class at Florida Hospital College of Health Sciences. I have one class left and it will have taken me 2.5 years.
Other schools that have less co-reqs may take up 18 months. Generally speaking it's 18 months to 2 years. ADN to BSN programs invariably know they are dealing with working RNs and are part time programs. Some like University of Phoenix and now my program allow you to double up on courses and finish faster if you choose and have the time.