Hey Gary - Here is what the SEOmoz help page has to say.
How We Generate Reports
The On-page summary automatically generates reports for any of your campaign keywords that rank in the top 50 of your primary search engine. The URL that it grades is the same URL that appears in the search results.
For instance, if you have 75 keywords ranking in the Top 50, you should have 75 On-Page Reports. This generation happens automatically within 24 hours of when your rankings are updated.
Here's the link to the page; it has a great video you can watch too: http://www.seomoz.org/help/on-page-reports
Ashley @ ScriptiLabs