When are potatoes ready to harvest?
Let the tops dry down on their own, and then wait another 7-10 days to allow the skins to thicken. This ensures the potatoes are injured the least from digging, and will store the best. If you want to eat the potatoes immediately (i.e. new potatoes), it is not as important to leave them so long in the ground, as the intention is not storage longevity.
Because we have two planting dates and several varieties in our community garden potato patch, we will have potatoes ready over a period of weeks. Hopefully this will allow for everyone to get a few potatoes. The guideline is that you can take enough for one meal at a time. Usually digging one hill will supply this.
Here are some links on potato harvest (and growing):
https://www.westcoastseeds.com/how-to-grow-guides/grow-potatoes/
http://www.homegardeners.com/potatoes.htm
http://blogs.vancouversun.com/2012/03/21/how-to-grow-potatoes-the-green-man-way-dos-and-donts/