Shelf life of homemade self-rising flour?
The shelf life is not a problem. This is one of the rare cases where mixing ingredients doesn't change their shelf life. All of them are shelf stable because of lack of moisture, and nothing is introducing moisture, so the mixture is shelf stable. 

You could, however, run into problems if you try to make a large batch and scoop out of it for every baking session. The reason is that this kind of mixture rarely stays homogenous. Every shake encourages it to separate, with the larger grains (salt, baking powder) gathering on top of the smaller grains (flour). If you store it somewhere undisturbed, the effect won't be too strong the first few times you use it, but you will be on the safe side if you pre-mix the quantity for exactly one batch only. 

As a side note, this seems like a strange way to go about it. If you either don't premix and just pour all ingredients into the bowl on baking day, or premix all dry ingredients at once (making your own baking mix, so to speak) you will be more efficient than just mixing a self rising flour separately.