Allowable flex limits for rigid FR-4

How much flex can a standard rigid FR-4 PCB tolerate before long-term damage occurs? I understand it depends on factors such as board thickness, layer count, and board size, but is there a thumb rule for estimating a safe minimum bend radius or allowable deflection? I’m concerned about copper fatigue or cracked traces when a PCB is mounted unevenly and forced to bend between screw points.