My "theory" is that learning a language is like learning any other discipline to a high degree of fluency. If you study a couple hours a week even if you don't forget what you learned in short order it's something which requires a period of immersion for you to be able to recall it as well as the day you learned it. To be able to expound at length on a topic at the drop of a hat you essentially need to dedicate a few hours a day every day. I think languages work the same way.