Is the user sitting on the same screen for 30 minutes? If they are entering data on a series of screens, the data should have been saved to the server. The only work the user would lost is the data on the last form that timeout, there isn't much you can do for that!

If this is an application that is design for long data entry sessions, you might want to consider making it a Winform program and send the data to the server via http in batch mode.