Originally Posted by bertie
There are some experts here who will likely chime in as well and give a better explanation but I think there is a point where the sole gets soft and spongy and that is the time to resole. If you poke it with your finger it gives a bit at this point and no longer feels hard to the touch. If you wear all the way through you may damage more of the inside shoe bits.
Nah, you only screw up the cork a bit, and that gets replaced anyway. as long as you don't keep wearing them after the hole forms, you're fine.
Resole when there's a hole, or if the toe looks like it's going to eat into the welt (the next layer at the toe, which you don't want to damage). In which case toe taps are a good idea on the new sole.