The ones I've seen in the farmer's markets (not many!) looked to be about 16-18" long, and they looked good.

I picked mine at 12-16 inches and they were good. I grew Red Noodle and they were tasty when the seeds just started to be obvious through the bean. Tasty like a snap bean. Later they were good when they were larger, more like black-eyed peas, but the outer hull had gone tough then.

I pick when they are about the diameter of a pencil regardless of the length.

Allow them to grow till color remains plush green.This will be seen till beans reach to 15 to 16 inches in length.Do not wait till the uniform green color begins to fade.At this time beans are of uniform thickness and of even hardness.Once color starts fading to lesser and lesser greenish going to whitish yellow the bean will lose the uniform hardiness - not good to pick up for edible use.Once it's green color fades bean will enter into seed producing phase.Once they enter into seed producing phase wait till color becomes pale yellow.Pick up some for shelling and allow rest to go for seed production which you can use next season.

Thank you all for your help. My yard-long beans are growing fast, although so far I have only 4.

Soon, I'll need a recipe, something easy and vegetarian. Does anyone have one they'd card to share?

We picked few yard long beans for the first time this season.I am posting the pic. of the beans we picked.
Yard long beans{{gwi:90170}}
I hope this will help you when to pick yard long beans.

I picked four when they were about 18 inches long and about as thick as a pencil. That was enough for two small portions. They haven't bloomed since. They just keep growing taller and taller!

Cook yard long beans the same as you would any snap bean or the snap beans you get from a can. You really don't have to cook them. Thai's eat as many raw as they do cooked. I just pick an eat while working in the yard. Pick at any stage of growth you like, experiment with what size you like best. Here in Korat Thailand we plant a new bed as soon as this bed starts bearing so that we can harvest year around. John