Powder will not rust the steel in an un-fowled barrel. You can leave it there all season.. The thing you need to concern yourself with then is whether or not the rifle will fire when you need it to.. The longer the rifle is left loaded, the more time it has for something to contaminate the powder charge and effect the way the rifle fires.

One time after hunting, I wasn't able to clean my ML for 2 days after 1 shot had been fired using 777 powder. I cleaned it thoroughly afterwards, and there was no damage to the barrel - it's still shiny like new. However, I wouldn't make a habit of it- better safe than sorry.

I use a solution of 50% Windex and 50% rubbing alcohol - works like a charm to clean the barrel. I don't ever use oil-based products to clean the barrel - only to clean/lubricate the breech plug, bolt, trigger, etc.