Fortunately toner is pretty easy to evaluate for yourself. And my Brother printer doesn't stop working when the low-toner warning kicks in. It also clearly shows that it's hardcoded to kick in at 1/3 capacity, they make no attempt to hide it. So I just buy a new one at that point, but I don't install it until the old one actually starts to visibly fail.

HEPA filter life is a little harder to evaluate using basic human senses.