They should be tender, but not mushy. Or open a bean with a fork resting the bean on a spoon. Barbara freezes mashed beans in 2-4 cup amounts for easy meals. Never add anything acidic, such as tomatoes, vinegar, wine or citrus juices, while the beans are cooking. Add these type of ingredients only after the beans are almost tender.
Acidic foods slow down the absorption of liquids.
When cooling, keep the beans in their cooking liquid to prevent them from drying out. When reheating beans, add a tablespoon or two of water. Presoak your beans overnight. Then you rinse them away and start with clean water. You can also rinse pre-boiled or partially cooked beans, though this can rob you of some of the nutrients.