Let's begin with the life span of a foundation. If your foundation has a pump then it will stay fresh longer, but if you are dabbing your fingers into a bottle everyday you should think of tossing your foundation makeup every 6-8 months. If you are prone to breakouts and are using the dabbing method, you should toss you makeup no later than 6 months. If you are using a pump foundation, it should be good for at least 8 months. But keep in mind that your foundation color can change at least twice a year anyway, so just get in the habit of tossing your foundation makeup every 6 months.
Face Powder and Blush/Bronzer
Powder face makeup can last over a year, if there is no shiny build up on the surface and you have not had any major skin irritations during use. Honestly though, most people won't be able to make a face powder last for over a year anyway. And as a general rule of thumb, if it smells funny or looks strange you should toss it. Powder blush and powder bronzers fall into a more extended category since most of us change our colors each season. With proper care, and clean brushes, you can squeeze up to 4 years out of your powder blush or bronzer.
Next, let's cover our eyes. Most experts suggest throwing away liquid eyeliner and mascara every 3 months. Personally, I keep my liquid eyeliner and mascara for at least 6 months and then open a new one. As for mascara if it isn't dry, clumpy, or smells foul then you should be okay for 6 months. You can gauge how long you need to keep something by the smell of the product most of the time. The only exception is if you get an eye infection; eye infections mean that you need to toss all eye makeup and start over again.
Powdered products, such as eyeshadow, will normally last for up to 4 years, as long as you wash your brushes regularly and don't reuse them after an eye infection. As for pencil eyeliner, it can last up to 1-2 years, if properly stored and sharpened regularly. Sharpening your pencils before each use will keep them clean, which will allow you to use them for 2 years or longer.
Lips and Nails
Finally let's cover the lips and nails. Nail polish lasts much longer than cosmetics and can last up to 2 years, unless you wear it with a fungal infection. If worn and an infection occurs, you need to throw it out immediately. Follow this same rule of thumb should your makeup come in contact with an infection, it should also immediately be thrown away. As for lipstick, lip gloss, and lip liner; they can last for up to 2 years, if you don't use them up first.
In the end, use common sense, and if it smells funny or looks strange it should be tossed out. As a general rule of thumb, change out your foundation makeup every 6 months, since your skin will normally change shades during that time. If you're using a product that contains anti-aging benefits please be advised that they will turn rancid quicker. Overall, if there is ever a doubt about whether to keep it or toss it, you should always just throw it out.
The best tip is to regularly clean and maintain your makeup items so they will last you longer, and keep you healthier as well. The last thing you want is some sort of eye infection or irritation from a mascara that has expired or an eye pencil that simply needs to be thrown out. This also includes cleaning your brushes at least every month with a little dish soap or baby shampoo.
Finally, if you have switched to natural makeup make sure you give the products a regular smell check, since they could turn rancid quicker because of the lack of preservatives. This is common in natural makeup which tends to usually only be good for 6-12 months, and replacing it often is in best practice to keep your skin healthy.