Originally Posted by
JanMM
Replace them when you no longer have complete faith in them.
+1. If you're riding in Florida I might wait until I could see cord, but if your hitting 45+ mph on downhills as we easily to on the tandem I use the above approach.
You will need to experiment. We lowered tire pressure on the tandem because we were wearing out rear tires very quickly. At high pressure (and depending on your weight) the tire you picture could be just on the verge or showing cord. At lower pressure, where the patch is wider, you may have reasonable miles left. .