Using Outlook Web Access and your session is timing out too soon. This may help

Change the settings as below and if you select when you login to owa using forms based authentication the following options:

-          “This is a public or shared computer” you will have a 5 minute session timeout

-          “This is a private computer” you will have a 60 minute session timeout

set-ItemProperty 'HKLM:\SYSTEM\CurrentControlSet\Services\MSExchange OWA' -name PrivateTimeout -value 60 -type dword
set-ItemProperty 'HKLM:\SYSTEM\CurrentControlSet\Services\MSExchange OWA' -name PublicTimeout -value 5 -type dword

Where "-value" (5,60) is the time in minutes.

Remember to restart Microsoft Exchange Forms-Based Authentication service ( MSExchangeFBA )

http://technet.microsoft.com/en-us/library/bb124787(v=exchg.141).aspx

http://technet.microsoft.com/en-us/library/bb124819(v=exchg.141).aspx