You can keep practicing to maintain your skills, or you lose them. Duolingo has been nice for quick maintenance study sessions that I would otherwise never bother doing. And the tracking remains useful because there are areas where I start forgetting.
I think the concept of the Forgetting Curve in language means that some knowledge you'll keep for years and years. For instance Duolingo thinks I'm completely rusty in one of my languages but I converse a few times a week so I've maintained it.
