how to know the time spent on webpage for every user

Look into Request, Load, and Unload events of an ASP.NET page. You need to write event handlers for these events and when page loads, you start a DateTime object for the current time and on Page Unload, you check the time again and find the difference between two times. You will also have to use Session states to store this time and use specific information because each user will have a separate session.

Here is more information on page events.

http://msdn.microsoft.com/en-us/library/ms178472.aspx