With Redtail's Calendar, you have the option to set your viewable time increments in 15 minute, 30 minute or 60 minute slots.
To set these, click on the Manage Your Account menu and select Change Site Preferences:
The second section you'll see on the Change Site Preferences page is Activity Management:
Click on the menu box to the right of Default Calendar Time Slots to select from your available options:
After selecting your desired Calendar Time Slots, click Save in the upper right hand corner of the Activity Management section and you will have successfully changed these.