Recently while implementing forms authentication for a WCF RIA Services application I ran into some trouble setting the expiration of the cookie that kept the user logged in. In the web config I had set the forms timeout to 120 minutes like so:
1: <authentication mode="Forms">
2: <forms name=".App_ASPXAUTH" timeout="120" slidingExpiration="true" />
This worked just fine. After 120 minutes of the browser being open without Silverlight making a request to the server, the authentication had timed-out and I needed to log in again. Once I had that working correctly ...