Joy Of Baking has an excellent article on how baking powder and baking soda work. It indicates (emphasis added):
Most baking powder used today is double-acting which means it reacts
to liquid and heat and happens in two stages. The first reaction takes
place when you add the baking powder to the batter and it is
moistened. One of the acid salts reacts with the baking soda and
produces carbon dioxide gas. The second reaction takes place when the
batter is placed in the oven. The gas cells expand causing the batter
to rise. Because of the two stages, baking of the batter can be
delayed for about 15-20 minutes without it losing its leavening power.
There are a very few applications using chemical leaveners that can be held longer (such as refrigerator muffins), but the batters tend to be much, much thicker, so that the generated carbon dioxide bubbles are trapped within the dough until baked.
The best way to have convenient access to something like waffle batter right when you want it is to prepare a dry mix of all of the dry ingredients (including the chemical leaveners), and then add the wet ingredients at time of preperation. Here is an example for waffles; or there is Alton Brown's pancake mix recipe.