If you want it quick and easy just read the card and you're good :). If you'd like a little more explanation with a deeper dive we'll first separate your personal care products into two main categories - wet stuff and dry stuff. Dry stuff lasts longer than wet stuff.
Liquid or cream products - those that have a water base - are going to rot at some point. Gross, but true. Generally at the point of manufacturing they've got a shelf-life of 14-24 months, depending on the product and preservatives used. And you're not getting it the day it was made - it has been sitting on a shelf, whether in a warehouse, in a store or in a box in a truck. Ideally, it hasn't been sitting a long time so you're able to get the proper shelf life out of it. Sun and heat also effect shelf life (as do steamy bathrooms and having items near the radiator). Oil-based products are an exception as they have a solid shelf life of 2 years since there's no water in them.
Dry products - pressed/loose powders (shadows, blushes, face powders) or dry masks that you mix with water have a much longer shelf life. They can last up to 3 years and this is when the smell test is an excellent barometer to see if you need to pitch stuff out.
When does the clock start? Technically it started when the product was manufactured, but that's almost impossible to know. You have two things you have control over - your "open" date and that little jar with the number in it on the label.
It's called the period-after-opening-symbol, or POA, and it identifies the useful lifetime of a cosmetic product after its package has been opened for the first time. So basically if you have a jar with "9M" in it you have 9 months to use it once you open it. In line with that, once you open your concealer in January, let's say, it doesn't matter if you didn't touch it for three months, it's still out of business as of Oct 1 (if it happened to be a 9M expiration). Make sense?
Here's the skinny for everything individually:
Including Foundations - Concealer - Cream Blush - Cream Highlight - Cream Eyeliners - Liquid Eyeliners - Cream Eyeshadows
1 year after opening
It's your face - if I have to advocate hard for you to be nice to your face we need to have a deeper discussion! Also, if you're breakout-prone paying attention to expiration dates can be the difference in clear skin vs. blemishes. Synthetic fragrances can often mask expired products which is one of the reasons I'm a fan of not using them.
Including Pressed/Loose Powder Eyeshadows - Blushes - Bronzers - Highlighters - Face Powders
Because they're dry you get way more wear out of that champagne shimmer shadow or hot pink blush. If they're loose they'll actually last longer as they have no binder. Pressed powder need a little something to bind them together which is usually an oil of some sort so they're a shorter shelf life but still pretty good. If you are over 30 and have shadows from college throw. them. out.
3 months y'all
It's your eyes folks and they're wet and so is your mascara - the combo can be a breeding ground for unhappiness if you're not throwing this out on the regular. I like to do it when I change my toothbrushes quarterly to make it easy - Jan/April/July/Oct. And even if you never wear it your clock starts the day you open the tube.
Including Lip Gloss - Lipstick - Lip Balm
So let's examine this because this one is hard for people. You are literally eating your lip product so if it's going bad, you're ingesting that. So if it makes you mad to buy a red lipstick because you're only going to wear it 4 times/year then buy a cheaper lipstick so you're not angry that you have to throw it out (or wear it more often!). You can wear it for two holiday seasons and then you need to replace it. Also, if it smells funny, pitch it.
Including Eyeliner Pencils - Lip Liner Pencils
You get a little extra time here with pencils which is lovely. My favorite sharpener is this one because it sanitizes your pencils when you use it.
Including all the things you think it includes - Cleansers - Serums - Eye Creams - Masks - Peels - Moisturizers
I'm sticking with natural products for this one and generally speaking you've got about a year which is why I'm a big advocate of using what you buy which sounds silly but think about it - how often do you buy something you love and then save it for special moments when you feel like you've earned using it. Often times it expires in the meantime and you could have used it up! Use it up :)!
This is your exception in skincare land - you get to hang on to your Argan, Tamanu, Jojoba Oils, etc longer than the rest of your skincare - they have a longer shelf life because they don't have water which would cause them to break down.
I'm separating this out because I think people think of it in its own category - if you bought sunscreen in May last year in prep for summer I would love for you to get a new sunscreen for this summer. Worst case you use it three extra months and then pitch it. HOWEVER, you should be going through 1 oz/application for the body and 1/4-1/2 tsp on your face/application so, if you're using it regularly you shouldn't really have leftovers. Again, use your stuff please!
Brushes are a great investment if you're nice to them. I wouldn't expect the freebies you got in your $35 purchase at the mall to have the same lifespan as investment brushes but good brushes can last you a while - I've got some in my kit from 10 years ago and they're great. Cleaning them is KEY to a brush's longevity.
Last but not least is makeup sponges - if you're getting single use sponges I'd throw them out weekly at best, if you're doing a Beauty Blender or the like it's three months.
Hope that helps!