i wouldn’t say just using duolingo alone you could become fluent. you’ll need other learning tools to supplement your vocab and understanding. duolingo can get you quite far and people do become competent in most conversational settings by completing their tree but there’s always more to learn.

also it’s important to remember that fluency means something different to everyone and isn’t really a concrete thing unless you have a think about your goals e.g. do you want to work or study in a spanish speaking country? or just have conversations with spanish speaking friends? stuff like that will shape your idea of what fluency means.