Magpie Diary (some of you may remember her as VegBitch) asked me how long a good nailpolish usually lasts, and after thinking about it, I realized that I have no clue.
My oldest (working) nailpolish to date is 4 or 5 years, but I have heard of people still using their old Sweet Georgia Brown-polishes from the 90's.
The only reason I don't have my old polish-collection is that I have moved one too many times to be bothered packing it up. If I had just realized that some polishes would be discontinued (like SGB), I'd done it in seconds. A good polish isn't hard to find, but your first Holy grail-polish is worth keeping.
The only issue I know of is that it can thicken and may need some nailpolish thinner, but other than that?! Nailpolish will outlive your socks, love for SATC and maybe even your love for nailpolish.
Well, that is, it may be all about toxicity - you may not want to use old nailpolish because back in the day, they used a whole lot of toxins we no longer see listed as ingredients in good polish.