Most would answer "never", and to be honest I have always been a bit skeptical of its benefit. I have read that you should always turn the image stabilisation off when the camera is tripod mounted but have never seen any evidence to prove it worth the effort. However, I was recently shooting landscapes on a tripod using extended shutter speeds (to get a nice smooth-looking sea surface) and could not get a sharp result. I tried one test frame with the IS off and noticed a marked improvement in the clarity of the file.
Canon EOS 5D MkIII +EF70-200mm f2.8 USM II lens, 30 secs @ F32, ISO 100 with the lens stabilisation turned On.
The resulting file is noticeably spongy...