from the Alexa site :

Answer

The overall Traffic Rank which is shown in the Toolbar, on the Overview page, and elsewhere in the service, is updated approximately every weekly. This is the main traffic rank for a site, and is based on 3 months of traffic data from Alexa Toolbar users. If you would like more information about how our traffic rankings are calculated, please visit About the Alexa Traffic Rankings.

Alexa also offers Traffic Details for each site. These can be reached from the site's Overview page by clicking the "Traffic Detail" link. The shorter term rankings (daily, weekly) and the traffic graph are updated daily. The "Today" ranking represents the last 24 hour period (midnight to midnight) for which we have data.