It depends how much you walk on them. There are two little plastic undercut ears that wear down at the front edge of the cleat (on the front corners), once those get too thin you should change the cleat.
"I haven't @#&$ed like that since I was an altar boy." - Hank Moody
“Bicycling has done more to emancipate women than anything else in the world.” - Susan B. Anthony 1896
"Genuine Campagnolo Spares - Inserts for Dura Ace pattern brake shoes." - PBK