Probably every battle or two. I know of no primary sources for this, so I am going to base this of reproductions and re-enactments. Shields were usually a form of laminated plywood, or planks. They were then covered with linen, leather or rawhide for durability and as a painting surface.

These shields would generally weigh only a few pounds, be relatively light yet sturdy, but would not generally be reusable. Most weapons of the time would do significant damage with each hit to the shield, and it would probably last through the battle, but it would not likely be in a condition to be used in a second.

For instance in the Norse duel called hólmgang, each combatant was given three shields for the combat, as the extended fight would be expected to destroy several; "Each contestant was to have three shields, and when they were destroyed then he must step on the cloak again if he had left it before, and defend himself with his weapons thereafter. He who had been challenged was to have the first blow. If one of the two was wounded so that blood flowed on the cloak, then no further fighting was to be done. If either one stepped outside the höslur with one foot, then that is called "he yields ground"; but "he flees," if with both. Each contestant was to have someone to hold his shield for him. He who was wounded hardest was to pay hólm-ransom (hólmlausn) -- three marks of silver (Hollander, Kormáks saga, 33-4)."

Edit:grammar