How Long Does it Take to Learn Spanish?
The following is derived from three years living in Argentina, Uruguay, and Chile, learning Spanish myself, and observing the experiences of countless other travelers.
These assume you are living in a country where Spanish is spoken.
How Long it Takes
If you do a 1 to 2 month Spanish course, stay with only Spanish speakers, avoid your native language, and employ as many tricks from How to Teach Yourself Spanish as possible: you will speak well after about 4 months, and be fluent after about 8 months.
If you live mostly with Spanish speakers, make an effort to speak Spanish as much as possible, and employ as many tricks from How to Teach Yourself Spanish as possible: you will speak well after about 8 months and be fluent after about 16 months.
If you live mostly with speakers of your native language (but take advantage of every opportunity to speak with natives), study casually, employ as many tricks from How to Teach Yourself Spanish as possible: you will speak well after about 18 months, and be fluent after about 24 months.
If you stay in "gringo" hostels or private apartments, go out with foreigners, never take classes, and don't study: you will be able to communicate basics after 6 months, and will never be fluent.
Remember: many, many people go to Spanish speaking places, stay for years, and leave with a very low level of Spanish. It is not guaranteed that living in a place will teach you the language, simply because "immersion" is rather easy to avoid in big cities.