when do you know when leeks are ready?

You can eat them at any size really, the plastic tube will blanche the stems. Aslong as the tubes arent tight on the leeks then you can keep pulling, Normally depending on variety, they get to around 2-3inch in diameter. Once the cold weather kicks in, they will happily sit in the soil as their grow rate will very quickly slow when the autumn winter cold kicks in.
Remove the tubing if its tight on the stems and wrap the white parts in thick paper or card (not to tight), just to keep them white

Thanks Nickyto8 they are Musselburgh I pulled three of various sizes.
prune