Session timeout defines when some session would be destroyed after some idle time.
If you want more control over sessions, you would need to use SQLServer mode, State Server mode, or a custom one taylored yourself.
I mean that, because, for example, SQLServer mode stores sessions in a standard SQL Server table, so, you can implement some SQL Server job, or schedule some task in Windows or in some Windows Service, so you can clean up sessions depending on your needs.
By the way, maybe you need to know that if you're using InProc, your sessions can be destroyed because IIS recycled application pool, or server got entirely restarted. And this isn't ending sessions, so the appropiate event and handler won't work in this scenario.
I repeat, if you need a better session management mode like ones I suggested you.
EDIT:
Check this page:
http://aspalliance.com/520_Detecting_ASPNET_Session_Timeouts.2
You'll learn how to handle session timeouts better, since Session_End isn't called when a session expires, but when you call "Session.Abandon".
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