Summarize this article:

Dried lentils should be harvested around 110 days after sowing. In most areas, this will be in July or August. Make sure that the weather is dry when you start to harvest. If you harvest the lentils during rainy or wet weather, they may not dry out properly. Lightly grasp the top of the plant before shaking it. If it is ready to harvest, you should hear the pods rattling. This rattle means that the pods on the bottom of the plant are mature and dried. Lentils mature from the bottom of the plant up. While the top pods may not be mature yet, you should look at the bottom for hard, dry brown or yellow pods. This means that you can start to harvest the plants. Don't try to wait for the top pods to mature if the bottom pods are ready. Most of your harvest will come from the lower pods. If you wait too long, they may shatter.

Summary:
Wait until July or August to harvest. Shake the plants to hear if they rattle. Inspect the bottom of the plants for brown and yellow pods.