I think that's mostly an issue of time. Personally, I find that I can keep up my speaking abilities in a language if I keep talking to a tutor for an hour every 1-2 weeks. So let's say I spent 30 years, learning a language every 1.5 years (let's say at an hour every day), and picking up 20 languages. If I wanted them all available at a very high level at the drop of a hat, I'd probably want to spend 10-20 hours every week doing maintenance with a tutor. So we're talking about 7 hours of weekly study (for each new language) and 10-20 hours of maintenance. So it's somewhere between a half-time and full-time job to keep them all fresh and continue learning stuff. (And you're paying tutors around $5-10/hr, so that may be an additional concern).

I think it's doable, just you've got to really love it or else you're going to burn out quick.

You can also look at this video of Alexander Arg├╝elles for his take on the matter, and he's wayyyy more dedicated to this stuff than I am. (Also I think he really prioritizes mastery, in all forms (written and spoken) of each of his languages).

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