While you can always change the length of an Activity during creation or when editing the activity, you do have two options in terms of setting a default length that will prepopulate your Activities during creation: those two options are All Day or 1 hour.
To set this up, go to your Preferences:
The second section you'll see on the Change Site Preferences page is Activity Management:
Click inside the box below Default to All Day Events and select Yes to set the default to All Day or select No to set the default duration for your Activities to one hour.
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.