According to About.com, whole asparagus spears should be blanched for two to five minutes, depending on the stalks' thickness. Martha Stewart recommends blanching asparagus for three minutes.
Asparagus is blanched in well-salted, rapidly boiling water. Removing the asparagus from the boiling water and plunging it into cold water stops the cooking process. About.com recommends cooling the spears for the same amount of time they were blanched. Blanched asparagus may be eaten as is or frozen and reheated.
Asparagus is prepared for cooking by bending the stalks until they snap apart where the tough, woody part turns tender. Martha Stewart recommends peeling the lower two-thirds of each stalk. The asparagus should be washed before cooking.