Every food item expires, even marshmallows, which don't seem like they would rot on you. While there may be truth to this, marshmallows don't stay fresh indefinitely.
Marshmallows stay fresh for two to six months when left unopened, though the exact time frame will vary depending on the manufacturer and the ingredients used to make them.
Marshmallows without preservatives have a shorter best-by date. They stay fresh for a month or two beyond the best-by date if they contain preservatives.
If you'd like to keep the marshmallows in top condition for longer, you could freeze unopened packs for up to six months past their best-by dates.
Open marshmallows should be okay for another week or two when kept at room temperature in a cool, dark place. They keep in the freezer for up to three months.
However, nothing is set in stone when going off of best-by dates. It's always best to check your marshmallows for signs of quality damage before you enjoy them.