From Touring Plans this shows the longest its been was about 2 months and always in January. Wonder why the September change and longer refurb.

Maybe im a bit more obsessed over this refurb because me and my family LOVE Splash and are sad to be missing it this go around.
"The following is a list of historical, current, and future closures (for refurbishment or other reasons) that we have recorded for the
Splash Mountain attraction at
Magic Kingdom in Florida.
Please note that this is an unofficial list and that future closures are very much subject to change.
- December 7, 2016
- January 10, 2016 to January 14, 2016
- January 5, 2015 to January 31, 2015
- January 6, 2014 to March 21, 2014
- January 2, 2013 to March 19, 2013
- January 3, 2011 to January 31, 2011
- January 3, 2010 to February 27, 2010
- January 4, 2009 to January 17, 2009
- January 1, 2008 to February 15, 2008"