I have a 40-year-old Martin with a bit of a belly. I put the JLD in there 3 months ago, took string tension off, and left it in the case.
It helps a bit, but it's not a miracle maker. In the pictures that follow, it looks almost unchanged, but there has been some flattening.
Before:
After 3 months:
FWIW, I haven't tightened up the device since the original install.