Building a language is expensive. Keeping one is not. People abandon languages they worked years for because they assume maintenance costs the same as learning, and it does not come close.
Speaking goes first, understanding goes last
After a few months away you will still follow a film and read a menu. What you lose is retrieval speed: the words are there, they just arrive too late to use. This is why maintenance has to include production. A routine of reading and listening keeps the part that was not at risk and lets the part that was decay anyway.
Twenty minutes a week is a real programme
One short conversation, or ten minutes of speaking aloud twice a week, is enough to hold a language that is already built. That is a fraction of what it cost to get there. Consistency matters more than volume here. Little and often keeps retrieval warm; a heavy session every two months does not.
Protect the specific things you will need
Maintenance is not general. If you use the language at work, rehearse work situations. If you use it with family, talk about family things. The vocabulary you will actually reach for is narrower than the vocabulary you own. Keep that subset sharp and let the rest sit.
Plan for the gaps in advance
Life will interrupt. A month off does almost nothing; six months off costs you a few weeks of rust, not the language. When you come back, do not restart the course. Speak badly for three sessions and it returns — that is the difference between maintaining and relearning.
A language you built is not fragile. It needs a small, regular amount of speaking, aimed at what you actually use, and it will stay with you for decades.