Every 2-3 months. You should never use your mascara until its gone. For several reasons. Every time you use your mascara it allows air in and dries it out. More of a concern than just flaky mascara is infection. While using mascara you expose it to bacteria which can then grow. If you poke yourself in the eye this can cause serious problems. When I was about 5 my mom accidentally poked herself in the eye with her mascara and unfortunately infected it and had to wear a patch on her eye while it healed. I’m sure no one wants to experience that. To be on the safe side I change my mascara every month. I also love fresh mascara. When you can get amazing mascara like Mary Kay Ultimate Mascara for only $15 why not change it often.