Harvest asparagus by snapping or cutting. Snapping is quick and easy. Simply bend the stalk near the base until it breaks. Snapped ends dry quickly so refrigerate or use soon after harvest. If you cut asparagus, use a sharp knife to detach the spears slightly below ground level. This base is woodier than snapped asparagus, so it doesn't lose water as quickly. Cut off woody ends before cooking.
So, how long can asparagus be harvested? Do not harvest at all the year of planting. The next season, harvest for 3 to 4 weeks or until the spear size drops off. Every year thereafter, the asparagus can be harvested for 6 to 8 weeks. (Ward Upham)