It was reported on the Google Webmaster Central blog by Vanessa Fox that the search results for Google Site Map users will be updated weekly. In the past, this only occurred every three weeks. Where a user could see what their average position was for the most searched terms that their site was represented in the SERPs for over a three week average. The stats they show are based on the queries that most often return your site in the results. Now web masters will be able to get fresher and more accurate data as the three week time frame did not account for any quick changes based on optimization that an SEO may perform. These updates will happen every Monday. However, the blog did not indicate any specific or set time.