A Google Analytics Help thread has confirmation from a Googler that your Analytics data will remain in your account for at least the past 25 months.
Anna from the Google Analytics team said, "data is stored for at least 25 months," when using Google Analytics.
After that, I guess you should export the data and save it locally. Is 25 months enough for you? Personally, I am not too sure. Here is a poll, let me know what you think:
Forum discussion at Google Analytics Help.
