I've been there as late as January 12th or 13th and the decorations at MK were still up. Some resorts had a bit, but most of the resorts had taken them down. If I had to guess, since Marathon weekend is around the 6 - 9th, that's when they'll come down...around then.
We were there this past January 7th (Monday before the marathon weekend) and visited HS, EP and MK--- decorations were still up in all 3 parks (EP felt like it had the least) and Christmas music playing in HS. Disney Springs also still had Christmas decor and the big tree... Don't recall seeing anything at the resort (ASM)...
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