If properly handled and stored pecans can be kept bright and good tasting for months, even years. Pecans purchased at a grocery store could be at least six months to a year old when you purchase them. Probably they were stored at the wholesaler's warehouse for several weeks where ambient temperatures are excessively warm. And, then they were stored and displayed at room temperature at the grocery store for several more weeks before being sold. This is why the pecans you purchase at a grocery store are typically dark in color and may taste "old" or rancid.
Pecans are normally harvested between October and March; because temperatures are usually lower at this time of year they may be held in-shell for several weeks without deterioration. However, once the pecans are cracked or shelled it is important that they be kept sealed and refrigerated, preferably frozen. Pecans kept at or below 0o F can be safely stored for two or more years. The color will remain golden and the taste will be fresh and pleasant.
A re-sealable, airtight container such as a plastic "freezer bag" or a glass jar with a tight fitting lid should be used to contain the pecans in storage. When you are ready to use the pecans extract only as much as you need at the time and return the remainder to the freezer immediately. To thaw frozen pecans move them from the freezer to the refrigerator until they are to be used. Do not allow them to warm up to room temperature until you are ready to use them.