There is an old saying that goes: Use it or lose it. Language, just like any skill, will disappear gradually if it is not regularly used. When I attended the university, I got Korean as an elective language course because at the time, Koreans have started to come to the Philippines. After six months, I could use basic Korean expressions accurately, ask useful questions and read Korean correctly (although comprehension takes time). Needless to say, my knowledge in Korean language was only good till the last expression I used with my Korean learners. Nowadays, I would only get refreshed whenever I train some Koreans or watch Koreanovelas (which I could only do when I have time.)
What should you do if you do not use your initial knowledge regularly? If you feel that your investment has slowly dissipated? Get a refresher course of the language for at least a week. Now, this could get tricky because one week is a short period so you've got to cover lots of grounds and make sure it's a quality investment. First, make sure you study under a native speaker. Second, determine if you want to stick to the basics again or you want to upgrade, meaning, go a notch higher, for instance, would you like your questions to be more heavy like involving comparison, making judgements, Do you want to be able to relate a story or deliver news? Do you want to improve in listening comprehension or reading comprehension? After determining your goals in the refresher course, create a program for yourself, with the help of your tutor. This program should be able to guide your learning process even when the refresher course is over. If you prefer to return to the basics of the language, you may still do and even use old materials, this way, mastery is achieved.