How long should I spend on Duolingo and what else should I do to increase my fluency?
OK ... how long is a piece of string?
Spend as much time here as you can easily do and absorb the learning. Whether it's 10 minutes on the app or an hour on the site, it all helps.
Past that, it totally depends on how you learn. If you are good at memorizing vocabulary, then Anki or Memrise. If you learn better thru podcasts, then many things on YouTube or Deutsche Welle. If you prefer to learn thru speaking, then tandems (even finding a remote one thru ConversationExchange ) will help. If reading is your thing to move forward, there are a plethora of German newspapers online (though Spiegel and Süddeutsche Zeitung are pretty tough) and Gutenberg.org had many online books. You could also search in your area for German / English books (the ones that have 1 language on one side and the translation on the other).
Overall, to increase fluency, I'd say just use it every day. Talk to yourself in German, write your shopping list in German, etc. It really does help.
Duolingo is a free app and you should feel free to spend as much or as little time as you please.
To increase fluency, you'll want to make sure you practice more often depending on your desires. I would also suggest using more sources as duo is awesome as a first step, but it is pretty much focused on reading and writing. To gain fluency, which is a measure of speaking (and listening), you'll want to use apps like HelloTalk, Tandem, etc as there are many such apps that you can talk with natives TODAY. I also love attending meetup groups (meet-up.com) in my city and I find the groups fun and super-helpful. If available, I highly recommend it. As I mentioned in the first section, how much time you spend is completely up to you. But be willing to try different techniques as time best spent isn't always here on duo. But good luck!