It is easier to lose an habit than to create one
If you are a runner you know that once you stop running you lose your training very fast. What it took three months to build up you can lose it in one month without training and, what is worst, when you re-start training it will take you again three months of training to reach the same condition you had before you stopped running. In other words, it is easier to lose your condition than to build it up and it is also easier to maintain your training than to build it up from scratch. That is why every person who runs regularly (or practices some other kind of sport) fears injuries.
It is easy to forget new knowledge
After few weeks that I am not practicing German I am noticing that I am loosing fast some of the knowledge I gained in the last months. It seems to me that something similar to what happens for physical condition may happen when we stop for a while to learn a language. In other words, we may forget faster than we learn. Why so?
Forming new habits and learning a language
I think this has something to do with the fact that learning a language, like gaining physical condition, is like forming a new habit and to form a new habit we have to go through a period when old habits (whether these are body habits or mind habits) are melted in order to allow space for the new ones. When you run new muscle fibres will form. When you learn a language new connections in your brain will be formed. This period of transition is crucial for the formation of the new habit and it is also the worst moment to stop learning because the new habit (the target language, or the physical condition) is still not “solidified”.
It is easier to maintain than to re-start from scratch. i.e. Do not stop learning before you reach fluency
It follows that the most efficient way to learn a language is to avoid gaps in your learning period before you reached fluency and probably even a while after that. Otherwise you risk losing months of work in relatively little time. If you need to slow down with your language training, do at least the minimum work to maintain your level as it is.
Fortunately, once you have achieved fluency in your target language, and you have practiced it extensively for a while, “black-out periods”, where you do not use the language, will not be as harmful as during the initial learning period.