ReportViewer timeouts in spite of timeout settings

set the AsyncPostBackTimeOut="" value in script manager

   <asp:ScriptManager ID="ScriptManager1" runat="server" AsyncPostBackTimeOut="56000" >
</asp:ScriptManager>

for more detail http://msdn.microsoft.com/en-us/library/system.web.ui.scriptmanager.asyncpostbacktimeout.aspx

Adding two of the config settings mentioned above did the trick for me.

In the web.config for the site set the executionTimeout = "10800":

 <httpRuntime maxRequestLength="2147483647" executionTimeout = "10800"/>

In the Site.Master set AsyncPostBackTimeout="56000":

 <asp:ToolkitScriptManager ID="smMaster" runat="server" AsyncPostBackTimeout="56000" />

Turns out that the answer is: There is actually nothing missing!

As it happens, the ScriptManager's timeout value is not persisted in ViewState (unlike the ReportViewer's timeout value) and the code was only setting it once inside a if (!PostBack) block. Fixed by setting the ScriptManager's AsyncPostBackTimeout property at each request (even postbacks). An alternative is to set it to a fixed value using the Visual Studio WebForm designer.