Hola, Belle,
Are you fluent in Portuguese? If you are fluent then I would say that three weeks would give you a fundamental base in spanish...but it really depends upon you and how easy languages are for you. I had noticed when I was in school that anyone who was fluent in Portuguese (there were a few here and there) that speaking spanish certainly came easier to them but where they had trouble was with grammar.
If you ask me (which I realise how redundant that sounds now, seeing how you did infact ask me :lol: ) I think Spanish grammar is very difficult. I also took six weeks of French in Nice, and while Spanish is easier to read and speak for the mere fact that how it looks is how it sounds....the grammar was harder then French.
Having said all that...if your goal is to go to Spain and while your there, learn some Spanish, I'd say go for 3 or 4 weeks (I'd recommend 4) however if your goal is to learn Spanish at an in-country Spanish school then I would say to go for 8 weeks or more....but again that depends on your goals....
Let me know what you decide to do...if you wind up going to dq, then you could say hi to some of the professors there for me....

Salud!
Michelle