Hi,
You can set this preference using Terminal, it is not available in iCal's user interface.
The command below sets the default duration of new events in iCal to 30 minutes. You should quit iCal before running this to see the result. Open Terminal, enter the following and press return:
defaults write com.apple.iCal 'Default duration in minutes for new event' 30
Then open iCal. New events entered by double clicking or pressing Command-N will have the default time of 30 minutes. You can set different values, but this change is global across all calendars in iCal.
To remove this setting altogether, enter the following in the Terminal and re-start iCal:
defaults delete com.apple.iCal 'Default duration in minutes for new event'
Best wishes
John M