[mdjlucan]

How do I know when it is ready to pick

[New shoot]

The silks (hair like threads that come out of the ends) start going brown.  While the cobs are on the plant, open 1 just enough so you can see a few kernels.  The should look whatever the picture on the seed packet showed - plump and golden yellow/bi-colour yellow and white.  It depends what you have grown.  Pierce 1 kernel with your fingernail and milky liquid should come out. 

If the kernels don't look ready or the liquid is clear, they need a bit longer.   If you have been careful, you should be able to close the gap in the leaves and leave your test cob on the plant to mature.