The symbol you’ve definitely seen on your makeup products, and why it’s so important.

When was the last time you replaced your mascara?

You may be holding on to it longer than you should.

According to Yahoo, New Idea’s fashion and beauty director Sally Nolan recently came across an Adelaide grandmother who believes using a 20-year-old mascara has made her go blind.

Unaware that makeup products like mascara have an expiry date, Shirley Potter contracted an eye infection after using mascara she had sitting in her drawer for over two decades.

She is now expected to fully lose her vision in the next three years.

Just as it’s not a good idea to eat expired food, expired makeup can expose your eyes, lips and face to bacteria and even infection, like in Shirley’s case.

Here’s a guide on when you should replace your beauty products.

The PAO symbol

Your number one guide is the Period After Opening (PAO) symbol.

The PAO symbol features on many products, indicating the product's exact shelf life after opening.

So if the symbol says 12M, it means that product has a shelf life of 12 months after opening.

Mascara

Mascara should be binned every three to six months. Yes, months.

While you may not have gotten to the end of the tube, it's best to avoid using mascara once you reach this point to avoid infection.

Oh, and stop pumping your mascara – it lets air into the tube, causing the product to dry out and creating a breeding ground for bacteria. No thanks.

Read more: Meghan Markle’s go-to mascara costs less than $10. Grab. Your. Wallet.

Eyeliner

Liquid eyeliner only lasts about six to eight months.

On the other hand, a pencil has a shelf life of one year.

Powder and Blush

Despite what you may expect, powder products can go bad too. Replace your powder-based products, like blush and face powder every two years.

Lipstick

You get just one to two precious years with your fave lipstick. So use it wisely.

If you can't remember when you purchased the lipstick, you can generally tell when it's expired if it has changed in colour, texture or smell.

Lipgloss

Just like the pumping motion with mascara can create a breeding ground for bacteria, the same can happen with your lip gloss. Lip gloss lasts about one year.

Foundation

Liquid and cream based foundations last between six to 12 months while powder foundation can last up to two years.

It's also important to regularly clean the tools you use to apply foundation.

Watch: Yes, you should also be cleaning your makeup brushes. Here's how to clean your beauty products.

Concealer

Similar to foundation, you'll get about one year out of liquid concealer and two years from a powder version.

Brow pencil

Your trusty brow pencil should last up to two years – if you treat it right. You should regularly sharpen the pencil and keep the cap on to avoid bacterial growth.