The way it works in Ner Yisroel is the BTL takes four years to complete. So if you learned in the Mir for two years, you can learn in Ner Yisroel for another two years and get a fully accredited BTL. I think Chaim Berlin and MTJ also grant a BTL that is fully accredited as well.
By the Association of Advanced Rabbinical and Talmudic Schools. It is fully recognized as a legal accrediting agency by the United States Department of Education.
A BTL, as Yeshivabochur said, will enable you to get into Law school, and SOME master’s programs. Not all. If planning to go into a master’s program, do some research first and find out from the school you plan on attending if they accept a BTL. Some don’t view it as a legitimate Bachelor’s degree.
Getting a BTL does not mean law school is the only option. I know of some who have gotten into master’s programs in the arts (like Jewish Education) and political science with a BTL. It is a worthwhile endeavor and have many advantages for the future.
I am unsure about how long you must attend to get the degree. A close friend of mine learned in Israel for two years and it took him two years to get his BTL from Ner Yisroel. I would assume every institution has different rules concerning this issue.