The shelf life of brown mascara, like any other mascara, is typically around 3 to 6 months once opened. This short shelf life is due to the fact that mascara is susceptible to bacterial contamination, especially since it's used near the eyes. Each time you use the mascara, you are potentially introducing bacteria to the product, which can then multiply over time.

The mascara's formula, which includes a combination of waxes, pigments, and oils, can also degrade over time. This can lead to a change in the consistency of the product, making it clumpy or dry, and reducing its effectiveness.

To maximize the shelf life of your mascara, avoid pumping the wand in and out of the tube, as this can introduce air and bacteria into the product. Instead, swirl the wand inside the tube to pick up the product. Also, always make sure to tightly close the mascara after each use to prevent it from drying out.

If your mascara starts to smell odd, changes color, or causes irritation, it's best to discard it immediately, regardless of its age. These are signs that the product has gone bad and may be harmful to your eyes.

Remember, using expired or contaminated mascara can lead to serious eye infections. It's always better to be safe and replace your mascara regularly.