That trendy bottle of nail polish you just bought will probably last far longer than your interest in the color. Nail polish is a chemical mix of fast drying organic solvents, thickeners including nitrocellulose, and colors and dyes. It's designed to last, as long as it's stored properly.
Unopened, your polish will last just about forever, according to Nails Magazine. Once opened, the nail varnish may thicken with time, but generally will last a year or two.
Once you've opened your nail polish, store it upright away from sunlight. Urban legend -- and some experts -- say polish keeps better in the refrigerator. An executive from nail polish manufacturer OPI cited in Oprah Magazine said the color stays fresh and doesn't settle if it's chilled. The same article also said a worst-case caution is that nail polish is flammable and could spark an explosion in an enclosed area. For a fail-safe option, use a cosmetic sorter that keeps the bottles upright on your dressing table.
Real Simple Magazine votes against keeping your polish in the fridge because it could cause it to thicken. It's bound to get thick and clumpy in time anyway, and when it does, just add a little nail polish thinner. When you're ready to dispose of your leftover polish, take it to a hazardous waste collector site. Or do as Orly executive Fiona Hay suggests in Pop Sugar Australia: Pour the leftovers onto sheets of newspaper, let it evaporate and then toss the papers.