In Lion, when you add events in iCal’s Monthly view they automatically default to all-day events. Since you probably don’t plan to spend all day at the dentist, you have to uncheck the all-day box and then enter the correct appointment times. There is a work around for this annoyance.￼
When you create the event, add the time of the event as part of the name.
That kills the default and enters an assumed time frame of one hour for your meeting. When you click on the edit button you can change the end time if needed and make any other changes you really wanted to make in the first place.