Welcome to the forum. It's good to have you here.

As for how long you can leave it loaded.. was the rifle fired at all or is this a fresh load? If the barrel had proper maintance and was fresh loaded it should not rust for a very long time. Granted all rifles need occassional maintance on the rifle and in the bore to keep them in top condition. Your rifle is no different.

As for plugging the barrel I never do that. Instead for long term storage I store them upside down, with the muzzle on a piece of soft cotton cloth. That way any moisture that might form will run down hill onto the cloth and out the end of the rifle.

Is there a reason you want to leave this rifle loaded for an extended period of time?