To keep your Marsala fresh for a longer period, it's essential to store it in a cool, dark place, like a pantry or cupboard. You should also make sure the bottle is tightly sealed to prevent air from getting in and spoiling the wine. And, if you don't plan on using it for a while, you can even store it in the refrigerator to slow down the oxidation process.
Sweet Vs Dry Marsala Wine at Megan Blackmon blog
But, here's the thing: even if you store your Marsala properly, it will eventually go bad. The good news is that it's still safe to use even after it's gone bad, it just might not taste as good as it once did. So, if you're a fan of cooking with Marsala, don't be afraid to experiment and find new ways to use it, even if the bottle is a bit old.