Hi there! I have a working fluency of Spanish. The best way to improve your Spanish is by emersion, like any other language. So, dive into it!
Don't worry. Spaniards are very friendly, and my experience was that they really appreciate you trying, even if not fluent. Plus, most people understand a little English.
There is really no set timetable for learning a language. It really depends on the individual. If you have a natural orientation towards learning languages, then it will be a lot faster for you. I think though, that for everyone, if you can let go of the fear and embarrassment, and really focus on "enjoying" speaking, even if not perfectly, then one can learn a lot faster, b/c you'll be more relaxed and your brain cells will not have an emotional mental block, you know what I mean?
If you'll be at an intermediate level, you really don't need to worry, you're at a perfect place for emersion. You don't have to profess to be bilingual, if you want to take the humble 'self-deprecating' approach, just say that you speak 'un poco.'
But you really have to reach out. For instance, one night I was hanging out in an Irish bar in Granada, packed w/ backpackers and foreign students. Everyone was speaking English. But I managed to meet two really nice Spanish grad. students, and we talked in Spanish, at times it was a bit challenging, but I had a great time!
(Sorry there is no scientific method/answer...)