It depends on how hungry I am. I normally start on them some time in July with the least healthy looking plants on the theory that they aren't going to be getting that much bigger anyway. I dig up only enough for 1-2 dinners and repeat as needed. Once they start to die back, I dig up the rest. I probably get a bit less this way, but I sometimes replant in July, so I'm actually getting two crops.
According to TMPL, it is 95 days. If MrsG started around the beginning of May, that would be about right. It could be the extra sun is moving the process along faster. It's also possible that they are a bit stressed from growing in containers- I always find it harder to keep plants happy that way.
Here's some fingerlings I dug up today. The vines had died back already, even though I think I planted them only 70 days ago in late May. The reason I say "I think" is that they were supposed to be leftover HoneyGold, but the shape is wrong. I think they could be some other leftover fingerling. Whatever they were, they were only OK. I just finished eating and the Purple Majesty I dug at the same time were much better.
They were good enough with the chicken without any butter. But I do love to cook them in butter"
Butter a cookie pan, slice them, and cook in the oven at 375 for about half an hour (variable depending on thickness), flipping once for the thicker ones. You can make them cook a bit faster by flipping each slice when you first put it in the pan to get butter on both sides.