Your aim is to get the batter into the pans and into the oven within about 20 minutes of mixing a cake batter. If you cannot use it all at the same time, you are better off mixing in small batches fresh for each cake. If you have leftover batter it should be refrigerated and brought to room temperature before using. Some people have used refrigerated batter up to a day old with some success. Generally, the texture of the cake will be effected.
With cakes such as Angel Food Cakes, the eggwhites can begin to deflate after about 5 minutes of whipping so time is important.
Once wet ingredients are added to dry, the leavening action begins to take place and there is a short window before the leaveners are effected and declining.