I would say that 1-2 years sounds right. I'm not sure because I didn't go that route, but I think the only real differences between ADN and BSN education is the extra classes that prep you for managerial roles. At my program, those would be professionlism, and leadership. Oh, also you'd have a research class.
As with any education, it really depends on how much time you can put into it. Would you be stuck taking only one class per semester/quarter, or could you take a few at a time?