While you can always change the length of an Activity during creation or when editing the activity, you do have several options in terms of setting a default length that will pre-populate your Activities during creation: those options are set within your Calendar Management and Activity Management preferences. To set this up, go to your Preferences:
The second section you'll see on the Change Site Preferences page is Activity Management:
You can choose between several different durations in the Default Activity Duration box: 15, 30, or 60 minutes. Note at the top of this section you also can set it so that when you create an activity it will be set up as an All Day Activity by default. Note: if you save your preferences to default to all-day activities during creation here, if you uncheck "All-Day?" during creation, the activities' length will default to the "Default Activity Duration" you select here.
After making this selection, click Save Preferences in the upper right hand corner of the Activity Management section and you will have successfully changed this setting.