When the pods near the top have filled out, you can start testing. Pick a few pods, boil for 5 minutes, cool immediately under running water, then pop the seeds out of the pod & taste. Harvest the plants when the taste & texture suit you. Ideally, you should harvest all plants in a row the same day; I wouldn't spread the harvest out more than a couple days.

If the leaves begin to yellow, harvest _immediately_. The seeds will be past prime, but most varieties will still be OK. If the seeds inside have begun to change color, you can still use them as limas; they make a great succotash, and are an interesting addition to chili (they stay firm & green). For chili, add the cooked beans near the end of the cooking time.

Note that once boiled or steamed, you need to cool the pods under cold running water, or they might end up overdone. The cooked beans need to be removed from the pods quickly, or they will absorb unpleasant flavors from the pod. Get a few friends or family to help you... once they taste them & realize they can eat as they work, it will be easy to get them to help again in the future. :-)