How long do you keep make up for?

Re perfume, keep it in the dark and it'll stay perfect for a long time. I found a 15 year old bottle of YSL Baby Doll in the depths of a drawer last year and it smells exactly the same as the new bottle I bought around the same time!
The only thing I'm really careful with is mascara, I replace it every 3 months or sooner if the smell changes.

Mascara I change every couple of months. I wear contacts and find old, dry mascara really irritates my eyes.

Pencil eyeliner I use until it's gone matter how old it is. I figure you're paring it all the time so you're removing anything really germy. I spray it with alcohol spray the odd time too and always replace the cap when storing it.

Gel eyeliner, same as above, keep using until it's gone. A make up artist once told me replace the lid in between lining eyes as all those seconds with the lid off add up and can make it dry out faster. If you check youtu there's videos too showing you how to bring dried up gel liners back to life. Goss make up has a good video on this.

Lipstick, again keep using until it's gone. Spray with sanitiser the odd time or wipe with a tissue to keep it clean. I go through lipsticks quick enough though.

Foundation, I usually use up a creme compact in 3 months or so. Finishing powders, blushers, eyeshadows, I use until they're gone, just spraying them with sanitiser every so often to keep them clean.

Cleaning your brushes regularly will also help extend the life of your cosmetics.