I see this stated in many places on the internet, but seldom referenced. The original claim was made in the book: 

Lebot, Vincent. & Merlin, Mark David. & Lindstrom, Lamont Carl.  1992,  Kava : the Pacific drug / Vincent Lebot, Mark Merlin, and Lamont Lindstrom  Yale University Press New Haven

Kava beverages are becoming more popular these days. Out of curiosity, I looked into the shelf life of premixed kava. It turns out there was a research article already published on this topic. Conclusion? 2 days. When you mix kava, 2 days is about the maximum time you want it to sit around.

Moral of the story? Don’t let your grog sit around; drink it down!