Ah that might be only for iis6.
If you can open the IIS console, you should be able to set it under the FastCGI icon under the features view.
Otherwise, you can change this in the applicationHost.config file. This is under C:\windows\system32\inetsrv\config folder.
Do a search for fastcgi, and you should see the activityTimeout value under that section. Give the server an IIRESET after making changes!