I think the thumb rule is that your game should be something new, not a game you did before. If you are doing a new game, but there are generic assets (code or not code) that you can re-use, than makes sense to use it. You will not learn more for re-implementing the same stuff. If your game is really different from your previous game and you can re-use an asset, then that assets is generic.
For instance, I always re-use (and improve/extend) player controllers when I do a new game. I also usually re-use and adjust the main menu, just replacing fonts and background image (sometimes adding/removing entries from the menu). I've often re-used light textures also. And so on.