Can you mix "all" of the dry ingredients to a cake and store them until you want to make the complete cake?!
If you stored it in an air-tight container, it would last a few weeks on your shelf. If you stored it in those vaccum-seal bags, like "seal-a-meal" bags, then you could store it for years, just like the store-bought mixes.
there are recipes for making your own cake mixes and they work very well.
And they last more than a few weeks.
IF the ingredients separately last months, mixed together they will also last for months.
Just as with anything like this, you want a nice air tight container.
If you want a homemade baking mix to keep on hand just google homemade bisquick and you will get plenty of results. allrecipes.com epicurous.com and food.com are all good sites.
The store-bought mixes are in vacuum-packed air-tight plastic and contain other chemicals and preservatives. I wouldn't store it for more than a few weeks if I made the mixture at home.
you did, and you can. haha
you are right