When the plant develops rust (goes brown) it is getting towards the point where it will stop growing and can be taken from the ground .. Exactly the same thing happened to mine and I dug them all up yesterday.
This year they seem to be undersized, none the less, grab a handfull of stems, tie them with a string and hang them somewhere in the shade with a bit of airflow to dry .. a shed would be good. Even though they are small, they will be fine to use in the kitchen.
For next years crop try planting them early (in Autumn this year) and this will give them a flying start.