If it's just decorations you are interested in (trees, wreaths, garland, window displays, etc.), I think those will be up in the 2 parks and probably at the hotels through January 6th. As for the ride overlays, It's a Small World Holiday will be running until (most likely) somewhere in the range of January 24th.
I have a strong hunch that Haunted Mansion Holiday may stay open through Sunday, January 10th -- although it has not been officially posted anywhere, it is likely that it will repeat what happened this past January. January 10th is also the last day for Aladdin, as you know, and (reportedly, though not announced by Disney publicly) the last day of the Big Thunder Ranch BBQ and the Ranch in general. So I suspect there will be some folks coming in for one last hurrah at the BBQ and to see Aladdin.
I have no information to support this, but I also have a strong feeling that the "Season of the Force" event (featuring Hyperspace Mountain) may begin right around that 'first week of January' time frame. This past January, Frozen Fun officially launched immediately after the holiday season (officially) ended (although HMH and IASWH remained open), so I have a feeling that Disney may try to do the same thing with Season of the Force, just to bring in people when the holidays end.