Application_End- exactly what you are looking for; The application is unloaded in accordance with the setting that you set, but by default it will continue to work for a certain period of inactivity after any requests are received or it will work while requests are constantly being received.
Please note that other things can cause the application pool to be updated and therefore cause Application_Endto be called; a certain number of recompilations (due to modified aspx files, etc.), a certain period of time, a certain amount of pressure in memory, etc. Again, they are all configurable, but are usually set to reasonable defaults.
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