You'll need to taste-test the lentils to check for tenderness after 20 minutes of cooking. (Rinse them off under cold water to help them cool down quickly; don't burn your mouth!) The age of dry lentils can affect their cooking time, so the timing might vary each time. Older lentils take longer to cook than fresher ones, and it’s nearly impossible to know how old they are when you buy them at the store. Adding salt or acid to the cooking water can also slow the cooking time.