We changed the session time out defaults as most users complained that the time was too short. Anyway, you could change it back through the web.config in this section:
<forms name=".ASPNET" loginUrl="~/sitefinity/login.aspx" protection="All" timeout="1440" path="/"/>
The timeout is set in minutes. Currently, it is set for 24 hours. Hope this helps.
All the best,
the Telerik team