Well, that would depend in large part on how and where you store them.
It's best to store all herbs and spices in airtight containers out of direct light and away from the heat of the stove.
Red spices (chili powder, cayenne, paprika, crushed red pepper...) should be kept cool, even refrigerated if possible.
Even under good storage conditions ground spices lose much of their flavour after six months, leafy herbs even sooner. Whole spices may be fine up to a year, but should be replaced when their aroma fades, since their flavour fades with it.
This may come as distressing news to those among us who are still using the spices that came in the rack they bought after moving into their first apartment in 1973. No harm done.. but you are missing out on a world of flavour. Clean up those jars, make a list and come on in and stock up on some full-flavoured spices at our self-serve bulk Spice Aisle. Pun intended.