If it's anything like other herbs, harvest no more than 1/3 of the plant at any one time. Beyond that it doesn't really matter too terribly much, unless you are concerned with the plants appearance. I personally would harvest the older growth leaving the younger leaves to develop more flavor.

Cheers!

Harvest when the leaves are big and flat. If they go further, the second set of leaves tend to lack flavor and are thinner and more like a fern. Then flowers appear as they quickly bolt. I have never been able to harvest anything after cutting off all the leaves that look like flat leaf parsley. These do not seem to grow back again and will just go to the next stage.