When they're of a good size. Sort of like supermarket kinds.

Must be done while they are a rich deep green. They're over the hill if color is changing to a yellow-green or yellow.

Not familiar with how big American Longs are supposed to get but in general for cucs pick them on the smaller rather than too long and fat side. Cucs that get too large will steal the energy from the plant and reduce your overall yield. Pick one now if it's good then continue to pick at about the that same size, if it's not good adjust the picking size.

If you don't think they are large enough to pick; wait a few hours and check them again. Its almost a certainty that you will know which cucs have the optimum development by the end of the first week's harvest. After two weeks you will be swamped with overmature cucs.

GrowingUp, length isn't such a good indicator. Especially since as the vines age, the cucumbers become shorter.

I was thinking of posting a series of pictures showing different stages of growth for American slicers - people could decide at what stage the cukes should be picked. Then, I could show the same cukes cut into slices so that the interior could be seen.

But, it would just be for fun. There are no right or wrong answers. Taste is subjective. You are the subject, how do you like your cucumbers?

I like them before seeds have fully formed, crisp, and flavorful. If your American Longs are like my varieties of American slicers, I'd be picking them before they were completely smooth and round - with just a slight degree of that immature ridging.

I hope you have enuf cukes to do a little experimenting. As Bmoser suggests, they develop VERY quickly and will soon become overmature (and stay that way ;o)!

Start picking them. You'll soon become expert at selecting the stage you like best.

Steve