This would depend on the specific statute you were charged with/convicted under. Assuming your DUI was what is called a "simple" DUI, then you would be eligible to apply for citizenship after your probation has been completed (assuming you qualify in every other way). However, because Immigration frequently interprets simple DUIs incorrectly as a crime involving moral turpitude (and also considering it may be a crime involving moral turpitude depending on the specific statute that applies to your case), it is in your interest to at least consult with an attorney prior to applying for citizenship to get advice you can rely on and be certain it is safe for you to apply for citizenship. If money is an issue, contact a local area legal aid or similar non profit offering free/low fee immigration legal services. Larger communities/cities frequently have citizenship events where you can get free help with preparing your application and talk with an attorney about your case (such as AILA's Citizenship Day, but many organizations have additional events throughout the year).
The 5 year saying is a time period that INS uses to determine your good moral character for N-400 applicants. Assuming that you have no other pending matters, you may submit your N-400 application after your 3 years probation from your DUI in 2012. It is then up to INS's discretion whether you have been rehabilitated. The more time that has passed, the better. Hope this helps.
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