That depends on several factors. How they're stored after you buy them is 1 factor (whether in plastic or in glass; kept out of sunlight in a cool place, etc.). The ingredients in them affect some more than others (for the more volatile ones...each time you open them, it could affect the loss of those components).
Kimberly (owner of OSDIY Shop) told me several years ago that there's nothing in them that can "go bad", as in spoiling. But the above could affect them over time, as far as how potent they stay. She estimated average time they'd likely be at their best would be 12-18 months. However, I personally haven't been able to detect any differences in ones I've had for even 3+ years so far. YMMV though.
