If the humidity is below about 35%, BP fouling will not cause rust.
If the humidity is above 35%, the fouling will absorb moisture from the air.
Follow Ed's post above, on wiping the bore and firing caps on the nipples before loading.
My guns stay dirty until I clean them. I've never rusted a bore & been shooting ML's since 1972.
One thing, never let a hammer down on the breach if the nipples are removed. The main spring will break out the bottom of the lock inlet.
After firing the gun, you need to clean it. Do not put that off until tomorrow.