its 1 frame every 5 seconds or 1 frame every 20 seconds.
Time of Projection: 30sec
Time of Event: 1hr or 3600sec
Projection speed: 18fps
If these were the parameters you wanted you would need a interval of 24sec. so assuming I did the math right you would probably want to select the 1 frame every 20seconds option
another way to look at it if you select the 1 frame every 5 seconds you will be able to capture around a 30min event. If you select the 1 frame every 20 seconds you will be able to capture an event between 2-3hrs.
edit:
I did the math wrong...figures. You would want 1 frm every 1 sec for a 1 hr event