Shadow9mm,
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The dezincification process is slow. I have a piece of LC brass that sat for months to maybe a year in a bag wetted with rain runoff. It has one pit under where it bloomed white with zinc oxide. It probed about 0.002" deep after all the oxide came off. It still shoots, but I wouldn't recommend shooting something like that, as you risk a gas jet that pits your chamber. I was running the experiment in a shot out barrel due for replacement.
There's a Canadian paper that used 0.1 molar hydrochloric acid (chlorine loves it some zinc) to demonstrate dezincification, but 24 hours later it still hadn't got deep enough that the affected layer couldn't be polished off. Also, it's action had slowed over that period, with most of the action in the first few hours. This is brass that was degreased and roughed up with a fine abrasive to insure good reactivity.
So it is hard to imagine much damage in normal cleaning time. Especially if the action slows as it progresses. Also, oxidation products tend to protect brass. Hatcher described putting polished brass and brass oxidized by the normal manufacturing and annealing processes on the roof of the Frankford Arsenal back in the 1920's sometime, when it was an industrial area with very corrosive air due to chemical plants (and probably from locomotive smoke). He wanted to see which could better tolerate that corrosive air and rain. A year later the polished cases were all eaten away, but the ones with an oxide layers were still intact. That's when the military stopped polishing cases.
So your darkening is probably protecting your brass. If your hot water is sixty degrees warmer than your cold water, figure it happens about eight times faster at that temperature.