I like Duolingo quite a lot, and different members of the household here, plus a number of students and faculty at my university, have had good experiences with several different languages.
However, it isn't ideal for achieving /conversational/ fluency - it's oriented more to translation skills, and it's best targeted for that purpose. It's also worth knowing that its system relies on implicit grammar instruction, which will work better for some people than others and, for trickier or less common issues, its algorithms can under-sample relative to what would be needed for users to get a feel for the underlying grammatical pattern. This matters more for some languages than others, but it's usually possible to speed things up by consulting one of the many free explicit grammar references online. For drilling common grammar, Duolingo is one of the best things out there.
Other things we've used to supplement Duolingo are:
Memrise, for vocabulary drilling - heaps of user-generated vocabulary lists, often in specialised topics as well as the standard intro stuff. The lists vary in quality - as all user generated content does - but it's free and there's lots to choose from. Duolingo caps out at 1500-2000 words for each language - Memrise covers far more (and you can create your own stacks if needed).
Babbel - this isn't free beyond a basic trial, but it's not terribly expensive either. It's more explicit in its approach than Duolingo, which means there's less need to supplement with other grammars. I don't think its drilling system is as good as Duolingo's for purposes of implicit learning of grammatical patterns, but it does offer more extended, contextual exchanges and will let you hear lots of different voices and accents, rather than Duolingo's one computer voice. I suspect the variation in voices would be more helpful for preparing you for everyday listening in a wider range of contexts. My son 11 years old) quickly found Duolingo too difficult, and prefers Babbel. I personally prefer Duolingo, but Babbel offers more advanced content than Duolingo does, at least in the language I've been working on, and so it's given me a way to move beyond Duolingo. I don't know if I would have paid the fee just for this purpose, since there are lots of other tools available, but since it's helping my son, and the subscription unlocks everything, I've been using it to top up.
I can't recommend specific resources for Spanish, as that's not a language we're working on in the household, but I'd also recommend scoping out online radio programs, YouTube channels, and similar resources in your target language. These are great for training the ear. In the language I'm trying to learn, there's a service that does slowed-down newscasts, some great systematic audio language lessons available for free, and an enormous amount of YouTube content freely available. There are also newspapers and other materials available for free online - a browser tool like readlang can be really handy for this - it links to a dictionary in the backend, and lets you click on words you don't recognise to get an immediate translation; then it stores the words you clicked on and makes index cards so you can review them later so that you recognise them next time.
Given your timeline, you might also want to pair up with someone online and swap conversation practice in their native language, with conversation in yours. You can do paid online lessons with experienced language teachers through a variety of services but, unless you need some sort of formal accreditation for your study, doing this for free through an exchange with someone else is usually fine.
