Assuming you are using 2003 (since you didn't include the version in your post), you can change this setting by going to the calender, right-clicking over the calender area, choosing "Other Settings" and then adjusting the time scale using the drop down menu. I find 15 minute increments to be a pain because the make it harder to see your day at a glance. In the future, it's best to include the version you are using in any posting here.
