Yep, I think I would learn to ride a bike first, before spending money on the entry level bikes that you find at the LBS --

Maybe, you can take 50 bucks of your budget, get on craigslist or local classifieds, or just ask around, usually someone you know has one collecting dust in a garage...grab a friend that knows how to ride a bike to help you find a bike that is moderately appropriate and doesn't need a ton of repairs-- I would definitely get a cheapie, just so you can practice in a large paved flat area... then when you have decent control, move up to your first good bike from the LBS.

How long on average does it take an adult to learn to ride a bike?

I do believe they make training wheels for adults though, or so I've heard -- they tell me bringing the seat all the way down helps too --

Well, good luck --