More digging around I found your response then on the 1858 Forum:
"I guess it depends on location. I live in south Ms. and I have left a gun fired with Pyrodex uncleaned for nine days without any rust or corrosion. I frequently go two or three days without cleaning. With Swiss I have to clean the same day. I left a rifle uncleaned overnight after firing Swiss and it had a white coating in the bore the next day. No harm done, it cleaned up easy but it gave me a start when I saw it."
My results with Pyrodex powder in my rifle left overnight go right along with dledinger's, and I lived in San Antonio, TX, which is nearly as humid as you I'm certain of.
A white film I wouldn't necessarily call corrosion, but the rust I found in my rifle the next day in my muzzleloader, and how hard it was to get it out will forever be in my mind. It wasn't but 12 hours or so. I would have been OK with a white film and not rust.