Shelf life is quoted as 6 months , 12 months, 2 years to indefinitely....take your pick...
Information:
One type of Single Acting Baking Powder is the combination of baking soda with cream of tartar - with a little cornstarch to keep the other two dry and inactive. [othertypes - phosphate and SAS single action baking powder]
SABP reacts when wet to produce gas. As it is "single acting", it works immediately. Liquids must only be added just before it is baked, or the gas bubbles disappear before it gets to bake.
This combination loses potency after awhile.
Double acting baking powder is a boosted baking powder. Cream of tartar is replaced by two other acids: one that reacts when wet, and another that reacts when heated. [It contains a little cornstarch also]
This combination also loses potency after awhile.
As all types of baking powder tends to lose leavening powder over time, homemade can be an interesting addition to the pantry.
Baking soda and cream of tartar have an indefinite shelf life. Keep in separate containers in the pantry, to make baking powder for an emergency.
Formulae:
1 ts double acting baking powder = 1/2 ts cream of tartar + 1/4 ts baking soda
[This works well if gotten get into the oven immediately.]