User Controls :: How To Insert Time Of Session End When User Closes The Browser

I have written a SP which maintains users log in and log out history,it works fine when any user logs in or clicks on log out, problem is that as per need its not to allowed  to keep the screen idle for 1 minute or more then in the log out column Null value is passed.

SP

Create proc usp_trackuserlogindetails
@username varchar(50) = null,
@command int = 0
as
BEGIN
if(@command = 0)
begin
insert into userlog (USERNAME,LOGIN,LOGOUT) values (@username,Getdate(),Null)

[Code] ....

Here when user manually clicks the log out button then 1(table above) is the output.

When user leaves the screen idle then after 1 minute the page goes to the Login.aspx page if the user tries to do something on the current page,here 2(table above) is the output can i store some hard core value instead of Null like Session expire or the exact session expires time.