I would think that once there is no more adjustment left on the chain anchors then the chains should be replaced. I got to believe they make them a certain length for a reason.
I have seen instances where a competitor will "lock out" a truck because there is no adjustment left on the free lift chains. In one instance it was because the drive tires were extremely worn and in another it was because the secondary chains weren't tightened properly causing the inner rail to hang down. In both of those instances we picked up a new customer.