Matthew Cohen/Rd.com No matter how many times you read how often to throw out beauty products (and what happens when you used expired ones), it’s hard to remember how long everything actually lasts. Plus, not every hair product is created equal, and some will last longer than others. That’s where this symbol comes in. The M stands for “months,” and the number indicates—you guessed it—how many months this jar or bottle will last. So a “12 M” product will last a year, but you can hold on to a “24 M” bottle for two.
Matthew Cohen/Rd.com Some products use an hourglass instead of the open jar. This one’s a bit less specific but still telling; it means the product will last less than 30 months. So if you’re staring at a bottle you got more than two and a half years ago, chuck it. And while you’re at it, make sure you haven’t hit expiration for these other beauty tools.