Even powder foundation shouldn’t be kept for more than six months if you’re being safe. 9–12 months I’m sure would be feasible but if you did that I would hope you are regularly sanitizing the makeup sponges/pads that came with the product or replacing them entirely. Your skin’s surface has bacteria that can grow in that environment, not much but enough that it’s not worth the risk as I stated earlier.
Like I said, regular pharmacy supplied make-up should be fin for her purposes, and pixie, I know it’s not what you wanted to hear, I almost felt bad answering, hehe. But, as long as you’re not like me and running through the stuff you probably won’t even notice the difference. Except in your budget. Besides I mix and match the pricier brands with the drugstor kind because a lot of it is really decent and pretty good quality overall.
If you’re curious I use cover girl true blend foundation, maybelline full and soft mascara and minral concealer, cover girl powders, and revlon eyeliners, brushes and eyeliner quads on the cheap end. I’ve tried everything the drugstore has ever stocked, and these are by far superior to the rest in my opinion. The revlon brushes are only about seven dollars and you will notice a major difference in how your eyeshadow looks when compared to using the sponges that are provided. You will really only ever need a flat contour brush and a round eyeshadow brush, they will work for you in any situation.
See how much more helpful I can be hen I’m not on a phone at work? :P