Have a look at the following two KB's

Browser disconnects users when they are inactive.

 Sage 300 Web Screens Adjust idle time logout session.

it is important that you are changing the setting in the <program files>\online\web directory    there are a lot of postings saying to change this setting that exists in other web config file but non of them work except the one in the program directory.  Thank you for the help.

it is important that you are changing the setting in the <program files>\online\web directory    there are a lot of postings saying to change this setting that exists in other web config file but non of them work except the one in the program directory.  Thank you for the help.