When users are working in CRM web shell they encounter specific log out time values when the application isn't being used that automatically logs them out of the application requiring them to re-enter their credentials. This typically occurs for users after 20-30 minutes and is determined by the values the organization has set in the web.config file. Users are finding though that when they log into CRM and simply close the tab that the application is running under in the browser that they can come back to the browser an hour later and re-enter the application by browsing to the URL without having to re-enter their credentials.
Download and install the latest patch, which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.