This approach lasted me until my vocab file was up to about 6.5k, at which point I realised I was learning new words faster than I could keep the old ones in my head, even with the help of my flash card program; and of the 6.5k words I had ever learned, I could only actually remember about half of those.
At this point, I reined my learning of new words severely in, and started going through my vocab file, with the aim of relearning every word I had forgotten (except for some I am likely very rarely to need, which I have purged from the file, the size of which has been rocking back and forth over the 7k boundary for months now). I started this process on the fifth of March last year, and have finally finished it today.
Of course, by now I have forgotten once again almost all of the words I relearned in the last year, but I am hoping that I might now have a few hundred more in my head than I did a year ago, and that continously relearning the words I have forgotten makes it easier to relearn them the next time.
I suppose the proof of that will be whether it takes me less than ten and a half months to go around the cycle again from now.
(FWIW, the 7k words I know are enough to cover over 90% of spoken German; it's the written language that pushes the lexicon size up.)
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