Walnuts are mature when the hull can be easily removed. they can be picked from the tree, or knocked down with a pole. However, they are usually not harvested until the rains have loosened the hulls until the nuts break free and fall to the ground. If wind knocks them off the tree before the husk looses, they will usually do so on the ground in a couple weeks.
Once the walnuts start to fall, pick them up quickly so they won't develop mold, discoloration, or decay. More info: http://ext100.wsu.edu/snohomish/harvesting-and-drying-walnuts-and-filberts/