I do not believe in nail polish expiration dates. Microbes typically cannot survive in polish, and the period after opening number on some nail polish bottles is merely a formality. They are there for governments or consumers who like them. Surely there are exceptions, but polishes that are well taken care of generally last a long time. Store them away from heat and light, and make sure you close them as tightly as possible after each use. Keeping air from penetrating the bottles is key. Bottles that are not tightly closed will go bad quickly, even if they’re brand new. Over time, some polishes break down faster than others. The brands I am mentioning in this article are guilty of this. Before tossing polish, it is always a good idea to attempt restoration. If your polish is so hard that you cannot pull the brush out, restoration gets a lot more difficult. Tossing those is probably best, although not necessary.
My nail polish collection is large. Polishes kept in good condition (away from heat or light) can last a long time, longer than the two years many people quote. A good thinner (I use Seche Vite) can also revive them. I have tossed polishes when thinner cannot save them or they have obviously gone bad, but it is rare. When I started collecting polishes again, I tossed all polishes older than the summer of 2013. To avoid ordering duplicates, I created a spreadsheet. Swatching them with these spoons I purchased from Amazon will help me compare the colors, finishes, and overall quality. All polishes only have one coat, because I wanted to spot the difference between my sheer and opaque polishes. We’ve all seen polishes that look dark in the bottle only to need 3-4 coats for opaqueness. No polishes have base or top coats either, because that can change the way the polish looks.