While I've been shooting one of my 1860s I've kept both my 3" barrel 1860 and my Pedersoli derringer loaded and unfired, occasionally carrying them on hikes or camping trips, for over six months. While winters in my part of Nebraska are dry, spring and early summer humidity levels average 80%.
I loaded the unfired 1860 with a Wonder Wad over 30 grs of 777. The Pedersoli has a screw barrel that allows you to pour about 10 grs of powder in a cup-like depression in the frame at the breech end of the barrel, place the .44 ball on top and screw the barrel back on. I keep the Pedersoli in the hollowed out book that came with the gun and leave the 1860 on my night stand (I live alone so I don't have to worry about others handling them), so neither are in an enclosed or particularly protected area. I recently shot them both and they performed just like they were recently loaded.
Not very scientific perhaps, but the fact that they both performed without problems gives me confidence that a loaded percussion revolver/pistol can be left loaded for extended periods and be a viable choice for home defense.