Cucumbers are usually ready to be picked eight to ten days after the first female flowers open up which is usually when the Cucumbers are the right size to be picked.
Leaving the cucumbers on the plant too long can cause them to have a bitter taste so picking them when they're ready is essential to having good tasting cucumbers.
Cucumbers are ripe and ready to be picked when they have a firm, green flesh and are around 6 inches long.