The major question is .... was the gun fired. If the gun was fired then it has to be cleaned. This includes a fowling shot for some, and even blowing through a couple caps or primers, although the primers do not seem as corosive as the black powder itself. For the protection of your rifle, if you have shot the rifle during the day then clean it that night.

If this is in reference to how long will the load in the rifle still be good... That is again influenced by the way the rifle was handled, weather proofed, and what it went through during the day. Then take into consideration what temperatures it has encountered during the day, and how it was or will be stored as to whether or not it will go off. This year I left a Black Diamond XR loaded for three days and it fired off fine... I normally shoot mine off at the end of the day, clean them, and then start with a fresh load in the morning. This year I was hunting further from home and walking out in the dark so I did not fire the rifle off......

Taking into consideration what weather conditions will do to not only a black powder rifle, but a modern rifle. How many times after hunting all day in a fog, mist, rain, whatever... you come in set the rifle up over the night and see some light surface rust trying to form. So again, if the gun was subjected to bad weather conditions, if you don't clean it, at least wipe it off and oil it.

As for how long can you leave a gun loaded... There are cases where a black powder rifle which was not fowled, was loaded and left loaded for years. When the rifle was finally checked they really found little or no damage from the rifle being loaded. The problem comes in when the burned powder has a chance to react with moisture and metal. It creates an acid of sorts that starts to eat away at the metal it has come in contact with.

Loaded rifles sitting on a mantal have been discovered to be not only loaded but very able to still fire after years of resting there. That is why black powder rifles especially the older ones we come across at gun shows and pawn shops and people's homes, where ever, should be handled with even more then normal care. It is too easy to take for granted they are empty only to learn they are charged. There have been cases where someone capped Grandpa's old musket only to discover that Grandpa left it loaded the hard way. I think we can all remember an incident last year where a person with an old black powder rifle was actingn stupid, capped it thinking it was empty, pointed it at I believe it was his granddaughter, and then shot her when he pulled the trigger and it discharged.....