What skills are you looking to preserve, specifically?
Comprehension is the easiest to preserve. You definitely don't need to use Anki for that. You can go for years (probably even decades) without using a language, and still be able to understand everything being said in it.
What is hard to preserve is the ability to speak a language seamlessly. I'd give it about two years of not practicing regularly, before you go from speaking a language on a native level to sounding awkward and constantly pausing, looking for ways to say what you're trying to say. I speak from experience, this happened to me with a couple of languages that I've been speaking for a long, long time. And it's not easy to get back, either. It's not just a matter of practicing for a few dozen hours...to be able to speak about complex subjects, you have to go through those specific subjects, not just everyday language.
Comprehension is the easiest to preserve. You definitely don't need to use Anki for that. You can go for years (probably even decades) without using a language, and still be able to understand everything being said in it.
What is hard to preserve is the ability to speak a language seamlessly. I'd give it about two years of not practicing regularly, before you go from speaking a language on a native level to sounding awkward and constantly pausing, looking for ways to say what you're trying to say. I speak from experience, this happened to me with a couple of languages that I've been speaking for a long, long time. And it's not easy to get back, either. It's not just a matter of practicing for a few dozen hours...to be able to speak about complex subjects, you have to go through those specific subjects, not just everyday language.