Pods should be harvested regularly, every couple of days, when they are approximately pencil diameter.
Green beans are one of the easiest vegetables to grow.  You can choose to grow a bush type or a pole type of green beans.  Pole types require support or trellising while bush types do not.  An advantage to growing green beans is that you can stagger plantings for a prolonged harvest.  Sow seeds directly in the ground (3-4 inches apart in rows) every two weeks from mid-May to late June for a continual harvest from late July into September.  Pods should be harvested regularly, every couple of days, when they are approximately pencil diameter.  Plants will remain productive for several weeks if harvested frequently.  There are many other types of beans that are relatively easy to grow as well.  One example is dried beans which grow much like a bush bean except they are harvested at the end of the season as the pods dry on the plants.  Dried beans can then be stored for months indoors.