How fast can a “forgotten” language come back to you guys

By forgotten I mean a language that you haven’t used in a longtime, maybe 1yr+ no contact.

German is my third language. I picked it up quite naturally and easily both from courses and 2hr/a day with flashcard and sentences building. I passed B2 after 9 months then moved to Germany. I lived there for 3 years and constantly got better at it.

After moving back to my home country I no longer have the environment to practice the language and was so busy with working 6 days a week from 9-6pm. Also I started learning Chinese as it seemed to become “must have” foreign language besides English. But after 3yrs I don’t think I’m cut out for it. I have very little progress with Speaking and Writing, while I can understand Chinese Series and TV show. I decided to temporarily quit this one and maybe try later to not stress myself out too much.

After 3 years not speaking a German word, I decided to get back to it and sign myself up for TestDaF (a test for German Skills at B2-C1 level). I did not prepare for the format of the test but tried to go through my old notebook that has a summary of all grammar that I learnt. After that I watch video on various topics like environment, technology, traffic, etc to try to speedrun my vocab and wake up old knowledge I used to know. My total prepare time was 2-month and I passed the test with 5,4,4,4, which can be accepted as C1. I’m very surprised with that tbh but happy of course. I’m now trying to consume more native German content to not forget the language again. Realistically I don’t think I’m at C1 level yet or as good as I used to be. But at least now I’m actively trying to progress again. I’m amazed that my brain still keeps those unused knowledge around and can be rediscovered when I want.