Cucumbers are usually ready to be picked 10 days after the first female flowers of the cucumber plant open.
Cucumbers require at least 70 days to produce cucumbers and be ready to be picked so they do require a pretty long growing season.
If it's been 70 days after you planted the cucumbers and you see cucumbers on the vine and they're firm, green and are around 6 inches in length then the cucumbers are ready to be picked.
The cucumber plant should produce between 12 to 16 cucumbers per plant and after 70 days the cucumber plants are usually done producing cucumbers after 70 days from the day you planted the cucumber plant.