How many levels does it take to speak Spanish?
You can do them all, but if you don't get much, much more practice speaking/listening from other sources, you'll hardly be able to speak at all, IMO. Duo compares itself to college classes. How many people do you know that have had 2-3-4 semesters of Spanish, and can barely, if at all, carry on a conversation? ;) I know many people like that.
Yeah, I'm noticing that too. Makes me think it'd be nicer if there was a place on the site here for people to do audio hangouts through Skype or Google Hangouts to be able to learn how to speak with real people during the courses. It would be really useful, especially if they break out the chat rooms by what level you're at in the lessons.
That would be really nice. However, the purpose of Duo isn't to teach you conversational Spanish. The purpose is to teach you translational Spanish. Despite the obvious trolls and porn, sites like Chatroulette can be pretty useful for practicing conversational Spanish.