Not a huge fan, feels too metagamey, like a once an encounter absolute defense.
A shield gets about 40 hit points, if the attacker misses you by 1, they hit your shield instead dealing damage. Axes do double damage. A loose translation of various medieval researchers found about the durability of shields in melee combat.
A shield with the rim edged with leather formed in a way so that it shrinks to hold the planks together and then faced with leather can survive about six axe blows. And considerably more blows from a sword or spear. Given the average damage of various weapons in classic DnD this works out to about 40 hit points.
Alternately you can use this from Judges Guild's Dungeon Tac Card from the 1970s.
SHIELDING (optional)
Shielding can only be performed with a shield (oddly enough). Your shield will absorb a certain number of hit points before being destroyed. After announcing that you are shielding in your round (by which you give up your chance to strike), you then roll for the following if you are hit by the opponent in his upcoming round (however, hits from behind are not affected by shielding). Roll 2 dice (dice type is based on your Dexterity rating).
GP Cost/Encum./Damage Pts.
Damage Pts. - able to absorb before being destroyed.
Large - Small
Leather 12/100/16 - 7/50/9
Iron 25/150/25 - 15/90/15
Wood 15/120/20 - 10/60/11
Wicker 8/80/12 - 5/40/7