I had the same problem, and found that there were a couple of environment variables that needed to be set.
One is SCONS_CACHE, which is a directory to store the cache in. e.g.
That sort of works, but there's also a default 1GB cache size limit, which wasn't big enough. Fix that with:
(Obviously you can set these in the environment variables in the Windows advanced system settings window instead).
Having done that, mine now builds entirely from the cache if there's no changes.