How Fast Language Fades

I took a Duolingo Spanish quiz to see where I stand. The results weren't exactly a confidence booster. Sure, I still managed to get quite a few things right, but there were doubts and uncertainties and errors. The truth is, I just don't flex my Spanish language muscle during my normal life. Every so often a couple times a year I have the chance, otherwise I just don't use it. 

When I was learning German in kindergarten I remember the teacher telling us that language is like a muscle and if you wanted to be strong like Arnold Schwarzenegger you had to practice (this was the early 1990s so terminator was big). Having studied Spanish for six years and spending two months in Costa Rica I feel my use of the language was pretty good, not having used it in years I am now rusty. The consolation I have is that it is easier to learn something the second time because you remember the difficult things to learn from the first time.