I don't bake as frequently as I used to, so if I decide to bake on some random evening, there's a good chance my baking powder has already lost much of its leavening ability — another trip to the store.
If I just keep baking soda and cream of tartar on hand, will they create "full strength" baking powder, even if they're left to sit around for months or even years (sealed, dry, etc)?
Baking Powder
For all practical purposes, yes. Just don't let moisture get near them.
Both are a single chemical compound which does not react with air. The only thing you have to worry about is entropy, and it will not do anything bad for the next few decades. It doesn't matter if you mix and use the day after buying or years later, unlike the mix, which ages because the components do react over time.
But you still will have suboptimal results when compared to commercial baking powder. In baking, you want a slow and even leavening action, which continues to happen after the cake has started baking. This is why today's baking powder uses multiple ingredients, some of which react at once, others at high temperatures. You cannot do this with your simple mix here. It will still work reasonably well (a century ago nobody had slow-release baking powder and they still made cakes), but you should try and see if the result is good enough for your standards. Also you will have to be more stringent with the baking directions (working very quick once the powder is moistened, not leaving a batch of batter stand around waiting for a free place in the oven, etc.)
You can certainly make your own from the recipe given and it will not begin to react until both ingredients are present along with water, but the issue is that the smaller the quantities, the greater percentage wise the error you get for small measurement errors.
Also, the recipe you have given is for a "single acting" baking powder which begins to expend its strength as soon as it is moistened; many commercial powders have a third ingredient, making them double-acting: part of the leaven occurs when moistened, and part only when heat is added, giving more and more reliable oven spring.
For these reasons, you will probably have better reliability and results with the commercial products.
Under perfect conditions, baking powder will store essentially indefinitely, as the components will not begin to react without the presence of moisture.
In the practical world, once the tin or container is open and begins to be used, small amounts of humidity will condense, and some reactions will occur, slowly weakening the baking powder over time.
Still, this is very slow. If you don't open it much, and keep it in an airtight container, it should last a very long time indeed, at least a year, perhaps more.
I would recommend:
You should get one, and probably several, years of use from it.
