Remember that more than click-through rate influences the quality of an ad: The quality of an ad is determined relative to other ads displayed at the same time by both its expected performance going forward and its historical performance. Historical performance data was not purged on February 5, but as time passes, the data generated after the launch of the new ranking model will have the strongest influence on ad quality calculations.
The time it takes to take both expected and historical performance into account to measure ad quality depends on the volume of searches related to a keyword. Keywords with lots of daily searches often generate enough data in a relatively short period of time until historical performance is a stronger factor for ad quality within that marketplace.
And no, the data used to determine ad quality is not “purged” at the beginning of a new month. Loren Baker has a comprehensive post about the fall out of the modifications. Read it here.