You do not want to leave Hoppe's #9 in the bore for any length of time. I would not beyond 24 hours, not even in a dry home environment, unsupported by oil immediately afterwards.

I always have Kroil laying around too. I used it in January to clean up a small isolated rust spot in the back-breech of my sidelock Shenandoah. Using Kroil, I can let it soak for a week, if necessary.