Well, since this hasn't been said, I'll jump in.
Black Powder is NOT corrosive. It is hygroscopic, in that it will absorb water over time and hold that water next to the steel in firearms. But, it is not corrosive and doesn't require immediate cleaning -- soon, but not immediate.
Substitute powders (Pyrodex, American Pioneer Powder, Black MZ, Triple 7, etc.) are all CORROSIVE. They produce by-products during combustion that IMMEDIATELY start to corrode any metal they touch. I have had brass and guns begin to corrode after using substitute powders by the next morning!!!!!!
So, giving, or taking advice from the perspective of one who shoots real black powder isn't useful to the person who chooses to deposit corrosive salts in their firearms.
I've found that guns have to be scrupulously cleaned before using substitute powders so they can be thoroughly cleaned afterwards. I've also found that any more than a few hours of letting those salts sit on cases or guns will start the process of corrosion. That implies, to me, that the corrosion starts immediately. I have found that ordinary cleaning with water and Ballistol does a good job of neutralizing the residue of substitute powders if carried out on a (sorry to repeat this) gun that was thoroughly cleaned to begin with and that has not had the salts on its surfaces for more than a few hours.
When I realized all this, it made real black powder much more appealing to me for cowboy shooting. Yes, I have to deal with fouling during a match, and I have to keep a close eye on the guns to know when they're getting grumpy, but I don't have to worry about corrosion.
Substitute Powders and Real Black Powder are very different things where corrosion of expensive equipment is possible.
May I refer you to the excellent post above by Professor Marvel?:
http://www.cascity.com/forumhall/index.php/topic,56362.0.html