I only use black powder and patched round balls, I've loaded in the afternoon, hunted left it loaded overnight and fired it the next evening with no misfires. If it's dry and cold I'll leave it loaded and in the cold for about 36 hours max. Other than that, I fire it at the end of the day, do a quick clean and oil. Next moring, I run a couple dry patches through it, fire off two caps, wait a minute or two (or blow down the barrel if you're in a hurry) to make sure there are no smoldering embers and load it up. I know folks that pull the ball (and I've done that if it's too late to see what I'm shooting at) dump the powder then fire a cap. It still makes quite a boom and flame because some of the powder is bound to get stuck on the lube. Could you leave it loaded longer than that without worrying about a misfire? Maybe, but I haven't tried it, down side with a flintlock or any exposed nipple ignition is there is air getting to the powder in the barrel (granted it's a very small hole) and black powder is very corrosive when it gets wet. With an inline you might have less of a problem because you can isolate the air to the nipple a little better I would think (never owned an in-line so I can't say with any authority). Can't help you on the pyrodex pellets never tried them.
Helicopter Bill