We gals amass quite a makeup stash over the years. But those compacts, bottles and other cosmetics you've been holding onto for years? They've got to go. Keeping old makeup is plain unsanitary -- even potentially hazardous to your health.Celebrity makeup artist Guy Guido gives us the scoop on what you can keep for how long and why.The ew factor While an overflowing makeup bin is fun to look at and dig through, you must part ways with the products you've had for a while. "Our skin, lips, eyes and lashes are full of everyday bacteria," says Guido. "If we knew what the bacteria on a mascara wand looked like after a week, we would probably run screaming."Product lowdown While there are no hard and fast rules about how long you can keep makeup around, Guido says that the consistency of the product is key. "The more moisture a product contains, the more it harbors bacteria," he explains. Therefore, you can keep powders longer than cream products.As for liquid products, Guido says that foundations in pumps are the least bacteria prone. "You are not constantly exposing the makeup inside the container to bacteria on your hands or on a brush," he says.In general, he recommends replacing cream and liquid products every 6 to 12 months and powder products every 1 to 2 years.Mascara is particularly important to replace. Guido suggests dumping a tube after 3 months. It dries out over time (making it ineffectual); moreover, the eye area is very susceptible to infection.While you should not keep makeup products around for too long, you also shouldn't share makeup because this makes bacteria accumulate even quicker. Guido says that, in particular, you should never share mascara, lipstick or lip gloss....but don't go overboard Bacteria are certainly gross, but we face them every day -- so replace old beauty products, but don't get too paranoid. Guido's parting words of wisdom: "We humans are natural bacteria fighters, so there is little cause to worry. If we were aware of and feared [all the] bacteria that is on everything we use, we wouldn't leave the house!"