Use the third page of the four page customize event dialog to modify the list of durations presented to end users in the event edit dialog. This form has two columns
If the end user chooses 'a short period of time' he would in effect be creating a 10 minute long event. If he chose 'a long period' he would be creating a 12 hour long event (720 minutes). You may use any text you wish in the As Displayed column. You must use a number in the Minutes column. As you make these changes save them and then access the event edit dialog and look in the duration dropdown to see the effect.