Black powder or substitute will not corrode your barrel in a short time. Only when it is fired and the residue left behind (fouling) is what is corrosive.
I, like FG, have left mine loaded - once for an entire season (4 weeks) and it went off without a hitch.
But like he said, if the weather is wet and damp, you may want to consider changing every day or every other day.As for the muzzle, if its damp, I usually just put a piece of electrical tape over it.