You should not extend the session - each user instance increases the memory usage on teh server, which can't be disposed of until the session ends. See MSDN:
https://msdn.microsoft.com/en-us/library/ms525473(v=vs.90).aspx[
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And bear in mind that each time the client communicates with the server, it resets the Session timeout - so if you are running out, it's because there is nothing happening as far as the server can tell!
If you are having problems with client session expiry, then I'd strongly suggest that you use Cookies instead - and let the session die a natural death.