I took lessons on and off for about a year and a half when I first got here in various forms before I got bored:
-- first arrived: 3x a week one on one conversation.
-- after that dropped to 2 x a week
-- then got bored so did UBAs intensive summer course, would recommend if you can get into a higher level as the classes were a good size, for lower levels may be tonnes of people. Summer courses were everyday for 3 hrs a day for 3 or 4 weeks -- did a January session one year, they offer them for February start as wel and then I think in March they go back to their 3x a week 3 month courses
-- then went to a one on one once a week for a couple of hours to do grammar...
-- then I got bored with the whole thing and haven't taken a class since 2007.
I read in spanish, watch the news, tv etc in spanish. But my work is all in English, and with my husband we speak a mix, so if we're not hanging out with his friends and family very often I do get a bit rusty (with my friends we speak a mix)
4+ years on and I have a strong accent still that I'll never lose, and I still understand more than I speak, and my written is pretty atrocious because I have an english keyboard so rarely bother with accents (only for año pretty much since it means anus otherwise and is giggle inducing for me, juvenile I know)
I'd definitely recommend some intensives to start, and then some one on ones... but at some point you'll get bored so if there's another activity you like, or a class you've always wanted to take, you may benefit more from doing that then just a regular class. Whenever you encounter a new situation (ie going to the dentist and you need to know how to say cavity) you end up having to study up a bit to figure it all out.