I feel like, if you have learned the language well enough, there are certain fundamental words and phrases that you will never forget because they are used so often. However, the lesser used details will slowly disappear if you do not use the language. This will also vary depending on what age you started learning the language. Chinese is my native tongue, and, even though I only use it now a couple of times a year with my family, I still know the language well enough to converse and read a little. However, I lost a lot of progress from Spanish, which I started in middle school, when I barely used it for two years.