There’s no single answer to the question posed by the title of this post. It depends on number of factors, including what hope to do with the language, how often and how intensively you study, which language you’re learning, and which language(s) you already know.
Some language learners find it helpful to set themselves targets and goals, such as passing a language proficiency exam. This gives you something to aim for, but also puts quite a lot of pressure on you. If you don’t achieve your targets and goals, or it seems likely that you won’t, it’s easy to become disheartened and possibly to give up altogether.
Here’s an alternative way of thinking about learning, from the Effortless Language Acquisition blog:
…learning is a lifelong process. There is no end. There is no graduation. There are no “permanent grades” or records.
The author of the blog argues that if you see learning a language, or indeed anything else, as a lifelong project – Constant And Neverending Improvement (CANI) is the phrase he uses, you won’t feel so stressed or worried. Instead you can just try to improve every week, even if it’s only by a small amount.