iPhone wifi connection log file?
Logs are available via Xcode, however they are overwritten pretty quickly in order to prevent from consuming large amounts of memory on the device.  Usually you can only get a few days worth of logs at most.  That means that the logs you are seeking have certainly already been deleted from the device.  These are the same diagnostic logs that are available at the Genius bar.  They are useful for troubleshooting current issues, but they are not usefully for determining how long you have had a problem.