Powdered sugar lasts pretty much indefinitely. Throw out the bag if you notice any signs of mold, pantry pests, or the powder smells off. Keep confectioner's sugar at room temperature, in a sealed bag or container, away from moisture and strong smells.

How can you tell if your sugar is bad?

If you see lumps in your sugar, that doesn't mean the sugar has gone bad. It just means that it's been exposed to a bit of moisture. All you have to do to use that sugar is break up the lumps, and take out a scoop, and never worry about sugar spoiling again.

Why has my icing sugar gone hard?

You can make your own powdered sugar if yours is so hard you can't rescue it. Simply put 1 cup of granulated sugar in a blender, add 1 tsp. cornstarch and blend until the sugar reaches a fluffy, powdery consistency.