I knew someone from Sweden who learned Chinese within 3 years, he had ADHD and said he spent an abnormal amount of time studying it, almost like an obsession. I am not sure what skill level he has, but he has no problem speaking / writing it and gets by in China. So I guess it depends on how much you want to learn it? If you are thrown out into a village in China, you will learn it much faster than say studying it for an hour a day :P