If this is something you make regularly, you can even make up extra dry mixes to have on hand. I do this often with the dry base for scones or pancakes. As a rule of thumb I figure the mix is fresh for as long as the expiration date for the baking powder/leavening agent hasn't passed. Past that date you risk a flatter finished product.
“Before you embark on a journey of revenge, dig two graves.” ~~~ Confucius