A look at the last 30 days of web searches for common malware infection indicators shows that the Internet Explorer Zero Day vulnerability has been in the wild since possibly September 12, 2012, or possibly as far back as September 8.
On September 11, the indicator search volume was at 67 on a sliding scale. As of September 16, 2012 (the last day Google provides search data for at this time), the search volume had increased to 94.
The search volume had peaked on September 2, and was on a fairly steady decline since, with the exception of a brief spike in search activity on September 8.