Default in Exchange 2010 you can only book resources and conference rooms for 1440 minutes (24 hours). Some times this isn’t enough for the users. When the user tries to book the resource longer he will get a message that the booking was declined.
The easiest way to change this is in Outlook Web App. Log in to your webmail with an account that has administrative rights. Click on Options.
Then select to manage Another User….
Choose the resource that you want to manage.
Go to Settings and change the maximum allowed minutes or just uncheck the check box limit meeting duration for unlimited meetings. Then press Save.
You can also change the settings using power shell.
Set-CalendarProcessing -Identity <MailboxIdParameter> -MaximumDurationInMinutes 2080
0 = unlimited
For more information see http://technet.microsoft.com/en-us/library/dd335046.aspx