I just googled "how long does it take to learn German", and found lots of funny and interesting comments, these from different people stood out:
Mark Twain said "Life is too short to learn German".
"It took me roughly four to six years. I've been here 29 years and still don't get the der, die and das."
"I married a German and moved here 11 years ago (okay, we did spend five of those years in the US) and I still can't keep up with Germans in conversation. Part of it may be that I've had to learn German and Bavarian at the same time, but still, the word order and the du/Sie thing do my head in. And really, how many words for "the" does any language need?"
"I really think there are as many rules as exceptions in the German grammar. I think learning German for four months required more brain power that getting my engineering degree. "