It all depends on your goals. Many people study a foreign language as preparation for travel abroad. Even if you just know a few words like “please”, “thank you” and “hello” the locals will appreciate that you're trying to make the effort. You'll get a warmer reception than if you just grunt and point, or worse, try speaking English louder and louder as if volume improved comprehension. Although “knowing just a few words” is a start, it is not the ideal goal. Baby-talking your way through a foreign culture might be charming at first, but if you need to explain to a local doctor about your allergy to antibiotics, baby talk will only frustrate everyone. If you are going to learn to communicate in a foreign language, your goal is to understand and be understood in almost every likely situation. How much is enough? When you feel that you can talk easily about those things that you would normally discuss with your stateside friends, you've probably got as much as you'll need for most circumstances.