You have "loads of time" even though you're in college? Are you sure you're putting enough time into your studies?

But seriously, your question is a little hard to answer. First off, not everyone learns at the same pace. Some people can just pick up things more quickly than others. So don't be discouraged if you don't progress as quickly as others. Take it at your own pace. Retaking the same level class at least once before moving up a level is a very good idea. Learning time also depends on how far you want to go or how good you want to get. You can probably learn enough waltz to do some basic steps and feel comfortable doing so out on a social dance floor in a semester. If you want to learn more advanced figures and how to do the basic figures better it will clearly take more time and practice. Having taken my ballroom dance classes in the college environment, I'd say that if you took one or two semester's worth of classes, you'd probably learn more than most people know. Not as much as the hardcore ballroom crowd, but more than the general public.

Try it out for a semester and if you like it, keep going. No need to make a long term commitment right now. As far as classes go, I would suggest starting out with a group class. It's cheaper and you'll learn the basic stuff you need to begin such as how to dance certain steps. You'll get a lot more personal attention and fixes for your specific problems if you take private lessons, but those are also a lot more expensive. Plus it's something of a general guideline that you don't take privates to learn new steps -- use the group lessons for that and then go to the privates to get the specific, detailed help you need for *you* to get it just right. Start with the group lessons and see how things go. If you really enjoy it and get hooked on ballroom, you can look into private lessons later on. I hope this helps.
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James Marshall
marshall@astro.umd.edu
http://www.astro.umd.edu/~marshall