Second attempt - still not quite sure how to know when they are ready for harvest, so going to risk the dark to see the light...
Researched this - when the cucumbers start to turn yellow and a little soft, you are past the prime harvest time. Ideally, the best time is based on size.

An advisor recommended harvesting once the cucumbers stop growing, to prevent them from sucking up all the plant's nutrients.
Harvested one on day 55 - looks, feels (not too firm, not too soft), and tastes great - used in salads and cucumber water. Yum!