How long is marshmellow good for? I made some a few weeks badk and it has been sealed in a zip loc bag.
It lasts for at least a few weeks in a plastic bag with no refrigeration. Open it up and see if it's getting slimy or starting to smell funny. If not you're good to go. If you want to keep it for more than about a month, you should wrap it tightly in plastic wrap and then in a ziplock bag or a plastic box with a tight-fitting lid and freeze it till you need it next. When you want to use it again, let it thaw thoroughly, unwrap it and knead it till you can work with it again. Some people recommend short bursts in the microwave (5-10 seconds) to get it pliable again, but you have to be careful not to overheat it because it will get runny and burn your hands, to boot. If this happens, just let it sit around till it comes back to room temp. I use ready-made fondant and have kept it tightly wrapped for many months - as long as a year - with no problem. The bucket says to use within a year of opening, but once I've colored it, I don't always need it that soon.