There was a test done on another forum in which three pieces of steel had a pile of powder, Pyrodex, Triple 7, and BP that were lit and left in a humid garage for 4 days. Pyrodex was by far the worst and left some pitting. The BP also had some minor pitting, but the T7 was nearly pristine after it was cleaned off.
I looked for it as I though I posted it here too, but didn't find it.
After taking two revolvers and a rifle to the range and spending much more time than I planned I only cleaned my revolvers and left the rifle 'til the morning as I had read a couple of CAS guys claimed to leave theirs for days without issue. I had used Pyrodex and it wreaked havoc on my barrel that took hours to get a clean patch from.
Later I read of a guy who applied Ballistol to the inside and out of his revolver and was able to wait with the theory being the fouling absorbs oil giving no way for moisture to touch the gun. I tried this and found it to be true. It's nice to know if you ever run short on time, though I clean my guns a soon as I get home.