How do you know when it's finally time to say goodbye to your favourite old shorts?
Is it when the stitching frays a bit along the seems? No. They're still hanging together. It'll be fine.
Is it when the chamois starts to get a bit lumpy? Nah. It's just molded to the shape of your bottom. It's not chaffing, so it's still fine.
Is it when you notice a slightly thinning patch on your inner thigh where your thigh rubs against the saddle? Oh no. That's just aero-dynamics...less resistance at your inner thigh.
Is it when you make a speedy dismount, desperately trying not to go pelting into the stream, but manage to catch that slightly thinning patch on your saddle, wobble and tumble a bit and hear that unmistakable sound of ripping lycra? Yes. It's probably time now.