My Benchmark DAC1 was bright sounding at first, then a little bit muffled and then open, smooth and more extended. It took about 100 hours at least to this point. There were still very small changes after that, I'm sure. My class D amp went to about the same changes but it was very slow - about 4x that (400 hrs).
As others have said it really depends on the unit and design and really more than anything the capacitors used. Also keep in mind as most Dacs get left on all the time it can help breaking in caps to cycle the until off and on from time to time.
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