I learned about 8 years ago. Took me about 5 months to get to where I could have a basic conversation, and about 9 months to what I'd say was about 95% fluency. (I've been told I am a natural signer.)
For those thinking I'm a jerk because my brother is 29 years old and I only learned to sign 8 years ago, he is not my blood brother. But we refer to one another as brothers, and that's how we see each other. I met Josh on the internet about 9 years ago. We both had the same outlook on things, a lot of common interests, like the same kinds of women (whadda ya want--it's often what guys talk about), and so on. We had been chatting on AOL and asked him what music he liked. He said none--he was deaf. My response was, for him, unusual. I said: "Really? That's cool!" All too often people answered: "Oh, I'm sorry."
Well, he went home for thanksgiving (he'd been at college) and I was bored at work (waiting for some project to get going and having nothing to do until it did). We chatted about 8 or so hours a day for two weeks, and just became really good friends. When he told me he took a book to thanksgiving dinner because only his mother really signed (and her sign stopped progressing past the time he went away to school at about age 12) I was just pissed off. When Christmas time came around, and some of the same topics came up (being bored for Christmas dinner) I remember I said, "look, you're like a little brother to me, and I promise you this--I'm going to learn how to sign, and when I get married I'll ask my wife to learn, and we'll teach our kids, and you'll always have family to go to for Thanksgiving where you don't need to read a book because you can't talk to people."
Well, I've not yet met Miss Right so I'm not yet married, and so have no kids, but I at least made good on learning to sign. After college, he moved up here to learn how to build computers so he could do something to make money while trying to write for a living (I am a published author, so said I'd help with that, too). After being fired from a computer job for essentially being deaf while on duty (the new manager there didn't like writing and didn't like that he couldn't answer the phone), I suggested he try to get a job at the Michigan School for the Deaf. He became a substitute teacher there, and eventually started working toward his degree in education.
I know I'm way off topic now, but I so often refer to Josh as my brother that to me, he is that--and yet we had different parents so I figured people shouldn't wonder. I don't get along with my real sibling, but refer to Josh as my little brother, and some other very close friends as my big brother and sister. To me, water is thicker than blood.