Eh I think skills are stickier than that. FWIW, I spent about 6 months doing almost no programming, but I didn't feel particularly unproductive when I started again.
Besides which, there's no reason people can't be continuing developing their skills in the remaining hours anyway. The 15 hours is just how much time you have to spend doing what you're told. And you can likely learn much more efficiently if you work with that intent instead of acquiring skills incidentally as your job requires.