With the default parameters, the MII session for a logged in user expires after 1h (60 min). There is also a default setting at the NetWeaver Portal level, which will log out users after 27.77 hours no matter how long the MII setting is set up. If a longer time is required, one can modify the parameter "SessionExpirationPeriod". IMHO 48 to 50 hrs looks the same as 27.77 hrs 
Do understand, that these sessions need to expire at some point.
This parameter is in the timeout value in milliseconds. In order to modify it there (the BASIS team will know), follow the steps below:
1. Open Visual Admin.
2. Select the Tab Global Configuration.
3. Select the Server Entry.
4. Select Services
5. Select Security Provider
6. Update the Parameter "SessionExpirationPeriod" and click update. Don't forget to click on the save button.
7. Restart the NW instance.
A working session might be lost also when a user's Internet browser preferences have been set to prohibit the reloading (and refreshing) of Web pages that have been previously visited and stored in its cache. If the browser prohibits the reload/refresh then session ID of web page will not be updated and it expires. For example, in IE do the following settings: Tools > Internet Options > Select the radio button 'Every visit to the page' or radio button 'Automatically' under the heading 'Check for newer versions of stored pages' After doing above settings close the browser window and re-start. Make sure that 'Never' radio button is not selected under the heading 'Check for newer versions of stored pages'.
Firefox might have the same type of settings, but unfortunately I don't use that browser.