When user is creating an appointment with more than duration specified in System Settings, user will get a warning message “The duration of the appointment is invalid”, In this case I would suggest decreasing the appointment duration or asking your CRM administrator to increase appointment duration in system settings.
Default maximum duration in CRM 2011 is 10 days, unless your CRM administrator changed to something else.
If you are the system administrator, navigate to Settings -> System -> Administration and click on System Settings. You can set the ‘Maximum durations of an appointment in days’ in the Calendar tab under ‘Set scheduling options’ shown below.
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