The one month mentioned for the return of the blood alcohol test was only an estimate and could vary a lot by virtue of many variables, including foul-ups at the lab or just getting lost. However, usually, if you don't hear anything after a couple of months, you may be in the clear of a DWI. I doubt if they will send you anything if your blood test was under the legal limit. Take the opportunity to say thanks for the "close-call" and be sure you don't drink and drive in the future - at all.
Until you know for certain, you should not rest easy.
Consult with a DUI defense attorney in your area. During your initial consultation, the attorney may review your alcohol consumption on the night in question to determine whether it is more likely than not that the results will be above the legal limit. If it was not above the legal limit, you may still be charged with DUI if the facts and circumstances of your case indicate that you were incapable of safely operating a motor vehicle.
Generally, you will not receive notice that your BAC was under the legal limit. Also, it is not a letter that comes in the mail, but the criminal charges. If no criminal charges come, then it would be fair to assume that you were under the limit.
However, you could still be charged with DUI under the incapable of safe driving provision. This usually occurs where there was an accident. If there was no accident, it is uncommon for one to be charged with DUI if they were under the legal limit. However, I have seen people charged where there performance on the field sobriety tests was extremely poor.
Finally, there may be a variety of other factors delaying charges. Once the charges come back, it is the responsibility of the arresting officer to file the charges. If they are on vacation or out on medical leave (typically for a work-related injury), the charges will not be filed until they return. There could also be a backlog of cases being processed by the magisterial district court.
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