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:
- Buy the smallest container.
- Store it in a small zip locked bag to keep out air (and humidity)
- Don't open it or leave it open when you are not using it.
You should get one, and probably several, years of use from it.