40-50 hours. You might get a little more out of it up to 100 hours, but the majority within the first 50 hours.
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I'm on my second 20x2-H (first one was damaged when TT was in transit). It gets really good after about 50 hours, but really comes into its own after 200+ hours when the top end really smooths out nicely. My new one has 100 hrs or so, and still can be slightly edgy on some recordings (violin music, choral music).