Looking at the documentation i found this information :
Data Extraction API requests are rate limited by number of requests and data download volume per unit of time (hour, day, week, or month). If you exceed the limit, a 403 error occurs.
Someone could tell me more about this limit ? How many call per day/month/year ???
Just spoke to webtrends support. The limit is 600 requests per use per hour or 1 Gig per user per hour.