You know Dele Test; that International Spanish exam?
I wanna apply for the Dele Test, but I don't know what my Spanish level exactly. If I were to finish the duoling's Spanish course of study, how would be my Spanish level?
If you reached level 25 on duolingo, I am willing to bet that you would be able to pass the "B2" exam at least.
As xoviat implicitly mentioned in his answer there are different levels at which you can take a DELE test as you can read over here: http://diplomas.cervantes.es/aprender_espanol/niveles_dele.html It is said that if you pass the highest level you know Spanish as good or better than native speakers. I would not dare to say that passing level 25 automatically means you will pass level B2. Passing level 25 is just a question of doing a whole lot of translations. What can help additionally is translating from English to Spanish on the "I want to learn English" website.