Don’t skip the 20-minute rest. Double-acting baking powder creates air bubbles twice: once at room temperature and once in a hot oven. For the best results, you should let batters with double-acting baking powder rest at room temperature to get the most benefit out of the first stage of leavening.
But how long to wait? It depends slightly on which baking powder you use. Check this page for in-depth info, but the short answer is that for Clabber Girl and most regional brands, 20 minutes is probably not a bad option. If you prefer a brand like Rumford, without sodium aluminum sulfate, two minutes is a more realistic target.
From a box sold in Columbiaville, Michigan.
Apple Sauce Bread
1/2 cup margarine or shortening
3/4 cup sugar
1 teaspoon vanilla
2 cups sifted flour
1 teaspoon baking powder
1 teaspoon soda
1 teaspoon salt
1 teaspoon cinnamon
1/2 teaspoon nutmeg
1 cup canned applesauce (lightly sweetened)
1/2 cup walnuts
Cream shortening till soft; add sugar gradually, continuing to cream together until fluffy. Add eggs and vanilla and beat well.
Combine and sift together the next six (dry) ingredients: add to first mixture and mix thoroughly.
Add applesauce and nuts and mix only until well-blended. Pour into loaf pan and let stand 20 minutes (if time permits) before baking in a moderate oven, at 350 degrees for 1 hour. Cool before slicing.
If applesauce is very sweet, reduce sugar to 1/2 cup.