Snap beans (Phaseolus vulgaris and its close relatives) are usually harvested when they are still young and tender and when they “snap” when broken. The entire pod is eaten along with the immature seeds inside. The color of the pod depends on the variety sown in the spring: it can be green, yellow or purple.
At this point, the pod is still thin and tender and you’ll barely be able distinguish the immature seeds inside. If you wait too long, however, the, the pod becomes fibrous and the seeds become bigger and take on a pasty texture. Also, if you leave the beans too long on the plant, it will stop producing new beans.
Ideally, therefore, you should harvest your snap beans when they are about the size and shape you see in the grocery store. Check the plants regularly, every 3 to 5 days, picking the pods while they are still immature, because this regular harvesting stimulates the plant to bloom again and therefore to produce more pods.
Runner beans (the climbing form of snap bean) will far outperform bush beans (the dwarf, non-climbing form) on this level: they’ll flower again and again — and will thus keep producing more beans — pretty much as long as you keep harvesting. Bush beans have a much shorter harvest window: you can rarely get more than 2 or 3 harvests per plant.
But the text above assumes you want to eat snap beans, that is, fresh bean pods. If your goal is to produce dry beans, either for cooking or so you can save the seeds for next year’s vegetable garden, though, you have to use a different tactic.
Dry beans are harvested when the pods are fully mature, later in the season. At this point, the pods will no longer thin, slender, and tender, but lumpy, papery and inedible. And you can no longer snap them. Wait until the pods have turned started to yellow before you harvest, This indicates that the seeds are mature. Don’t wait until the pods turn completely brown, though, otherwise the pods they may open up and the seeds will fall to the ground.
When you grow dry beans, you won’t get a prolonged harvest: they’ll all be ripe at about the same time.