HM usually goes down for about 2 weeks to the remove HMH layover.
As for SW, if the rumors of adding the Disney characters throughout the ride are true (please no!), they are doing that during the post-holiday closure as well as the holiday layover removal. Remember, IASW was down for 8 months with a hard deadline of re-opening in time for the holiday season. If they have more work to do on that, they will be doing it when they close it again to remove the holiday layover.
I also want to add that I have my favorite rides (one was down for 8 months

) and things that were on the top of my list when we visited WDW, but never once did I check the refurb schedule when picking our travel dates.
I adore the Country Bears Jamboree and was SOOOO excited to be able to see it again when we went on our once-in-10-years trip to WDW, along with riding WDW's version of IASW. Was I heart-broken when I discovered my bears were closed for holiday decorating when we were there? YES! Did it in anyway make our trip less somehow, no. It's 1 thing out of 100s to enjoy. It IS possible to have a wonderful vacation when the 1 thing you really wanted to see is closedd for refurb.
Keep in mind, you could plan based on calendars until the end of time and something mechanical could happen prior to your trip that would affect 1 attraction that you have planned your entire vacation around.
I just hate to see people plan their entire vacation around 1 ride. It is not the end of the universe if you don't get to ride X. Trust me, I have a great picture of the front of the Country Bears Jamboree - and a hope that they will be there in however many years when we go back!
Just my perspective,
Dreams