I chuck my mascaras when they cease to go on well. That means some products get chucked after six weeks (and never bought again) while others - like my Chanel ones - I'll use until they're done, which might take 4-5 weeks. I reduce wastage by only having one mascara on the go. Ideally, I prefer to use travel-size samples, as they are also portable.
Liquid eyeliner I am scrupulous about, because it's even closer to your eyes than mascara. As soon as it starts to look 'stale' I get rid of it. Two-three months. Pencil eyeliners, I am religious about freshening by sharpening and sealing into their lids.
Nail varnish, I chuck it when it separates, though I do have 'double standards' - I either buy totally cheap stuff for the mad colours (from Barry M) or one must-have shade every season from Chanel, in which case, I keep the latter sometimes simply because the product looks great, and never go back to using it.
Lipsticks and lip gloss, I'm afraid I don't chuck out much, but I don't suffer from cold sores, so I don't feel that they're that unclean, as long as they're confined to my own mouth.
Eyeshadows. God, I have some antiques. Somehow, I never associate pressed powder eyeshadows with anything that could 'go off'. I see it as largely inert stuff.
So a mixed bag there, with the old and the new.