I guess it is difficult to quantify as there is not a clear routine and set time.
I am currently learning Japanese, and I felt the same way. When I stopped studying the language, I forgot everything I just learned. But I think it only happens to beginners. As a Japanese learner (very beginning), I think I need to study it every day, but as for English, I don't have to study it on a regular basis. I just read news and watch shows to maintain the familiarity toward the language.
Well I study English every day - on the way to/from the work (approximately 2 hours) - I read smth in English, listen to broadcast or watch videos.
After the work I haven't enough time for studying English.