When baking powder has been opened the opened baking powder will last for up to one year.
Baking powder has a shelf life of up to one year and baking soda is sensitive to moisture and humidity and if the air is moist or humid then the baking powder may only last a few months to 6 months.
But if the moisture and humidity level is low then opened baking powder may last up to one year.
The baking powder remains safe to use after a year but it gets hard.
So you could break the baking powder up and still use it after a year of it being opened.
But it's best to buy some fresh baking powder for baking baked goods with.
Older baking powder may not bake the food properly so it's best to use fresh baking powder.
You could put the opened baking powder in a sealed container after opening it to help extend the shelf life.