When do spices begin to lose their flavor?

Spices will lose their flavor at different rates depending on the particular spice involved. I would recommend throwing away any ground spice that are over a year old. Whole spices (cloves, nutmeg nuts, etc) will last near indefinately. A few things to remember:

1) Air tight, waterproof containers will maintain flavor the longest. They will also keep weevles out of your spices (yes, you can get weevles in spices as well as flour.)

2) You can account for "fading" spices by just adding more, though this takes a cook who is familar with the dish and isn't afraid to double the amount of a particular spice. (I've had problems when I get used to doubling the curry in a recipe and then buy new curry powder and forget to reset to the original amount...)

3) Buying bulk spices usually ensures fresher spices in smaller quanities, so you don't waste as much money throwing away old spices. But you need to be careful of trying to get a "fresh" spice that's been sitting on the shelf for 3 months - look for signs of the store's bulk spices having high turnover.