I recently had the opportunity to work on a corporate film project for a local company. On set, one of the ladies asked about her flaking mascara. I asked her how long she had had her mascara and she told me over 3 years! I began to explain to her the dangers of using expired products and I was not surprised at all when she said "Oh, I didn't know that"
I feel like thats the case with many people. A lot of us don't know how long the shelf life of our makeup is and why its so important to discard makeup after it has expired. Since makeup companies are not required to post expiration warnings on their labels, you have to use your best judgement to know when its time to toss it out. The preservatives used in most cosmetic should kill common-use bacteria. However, studies have shown that a small amount of bacteria is often present in makeup prior to purchase. Upon the makeup’s exposure to air, airborne bacteria are introduced. The concentration of bacteria is augmented with unsanitary practices such as unclean hands or unclean applicator or brush. Don’t be alarmed; this is usually not a health hazard. If you are healthy with intact skin, a small amount of bacteria is not a problem. However, there is a higher risk for infection from contaminated products if your skin is non-intact. So if you suspect an infection don't use your makeup. If you have used it while you had an infection get rid of it and NEVER share your makeup with others.
As for the shelf life, heres some professional advice on how long to keep it around before its time to toss it out.
As for mascara, It is so important that it is replaced every 3-4 months. This is one you definitely should not keep around longer!
And one last thing... Never add water to formulas that are drying out. Your just asking for problems at that point! If you have to add water, your way overdue!!