An official response would be good for this.
How often do you visit landing pages and update Quality Scores?
[adwords.google.com...]
Quoting from that 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.
AWA