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

As Displayed

The first column contains the text that the end user will see in the duration dropdown on the event edit screen. For example it could say 'a short period of time', 'a medium length period', 'a long period'.

Minutes

The second column contains the number of minutes represented by the first column. Continuing the example, the number of minutes for each of the preceding examples might be 10, 60 and 720.

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.