Just a factual post, based on a DigitalPoint Forums thread. How often does Google update the AdWords Quality Score value for your ads? The answer is addressed by GuyFromChicago, who links to a Google Help page.
The AdWords system visits and evaluates advertiser landing pages on a regular basis. Due to the evolving nature of the quality evaluation system, the frequency may change based on improvements to the system or the level of user traffic to AdWords ads and landing pages. We plan to visit pages at least once a month but may do so more often.
So at least once per month.
Forum discussion at DigitalPoint Forums.