I tend to replace my mascara after a couple of months as I have quite sensitive eyes. As long as they don't look as if they are separating or smell funny other things run out after about six months anyway. Apart from lipsticks which I think last a couple of years.
I am a bit fussy about applicators, so keep those clean and wash brushes and sponges often.
I agree, the beauty industry does perpetuate this churn of stuff. But then a lot of it is smoke and mirrors. Every week there is a new type of something, bigger and better.
I tend to keep things simple, and use the products that I know work for me.