I love France, I love the food, and the country. Unfortunately, I have a problem with the language. All I have is two years of highschool french. My last trip to Paris, I was able to communicate on a basic level. Anyway, I live near Boston-and the Alliance Francais offers courses. Are the Pimsleur tapes better? I’d like to attain a “comfortable” level of conversational fluency. Any french speakers care to comment? 
I’m not a French speaker, but I managed to get pretty decent proficiency in Portuguese (I’ve probably lost it all by now, but that’s beside the point.) I hope some of my experiences in learning a second language could be of help to you.
I’d strongly suggest you take a class–even if it’s a lousy class–and maybe use the CDs as a supplement to your coursework. The main use of language is to interact directly with other people. Any situation that lets you (or, better yet, makes you) speak with people in that language is going to help more than listening and writing alone would.
Second–it helps if you seek the language out. Can you rent French movies at your local Blockbuster? Are there simple books (try cookbooks, or children’s books to start) in French at your library? If there are old French texts at your library or in used bookstores, you can pick one up and scan for poems and simple short stories. Can you listen to music in French? The more contact you have with the language, the better off you are.
You might want to put up ads in various math, stats, and science depts. in the universities closest to you, offering to trade an hour of English conversation for an hour of French. There might be grad students who speak fluent French and want to improve their English.
If, when you go to France again, you find that you have trouble holding casual conversations, don’t be discouraged. Casual conversation, especially at parties or clubs, can be pretty hard to get comfy with. If you’re willing to risk it, and be in as many conversations as you can, you’ll find that eventually you do pick it up.
Oh, and, IME–once you’ve got the grammatical basics down, and you have a semi-decent functional vocabulary, if you spend about 3 months in France (or somewhere else with lots of monolingual French speakers) you’ll find that your brain has switched to French and that you’re highly proficient in the language. In fact, you might find it hard to go back to speaking English at first.
How long it takes to learn the basics–grammar, a decent vocabulary, common idioms, etc–depends on how much time and effort you put into it.
I went to classes at my local Alliance Française for a year (two nights a week) several years ago, working my way from beginner to moderate etc. At the end of that I had a reasonable fluency. When I visited Paris soon afterwards I was pleasantly surprised at how well I was able to converse with locals. Unfortunately I have slipped a lot since because I haven’t been going to regular conversation classes.