Time-lapse photography is a technique at which film frames are captured at much lower frequency than used to view the normal sequence. When played at regular speed, time appears to be moving faster and thus lapsing. For example, a 30 minutes event may be captured once every 2 seconds, thus generating a 30 seconds video clip which is played back at 30 frames per second (fps).
Based on what you’re shooting, you’ll want to know how long your actual event will last (or at least, how long you’re willing to shoot for), whether you want your final movie to be blocky or smooth, how long you want your final movie to be, and based on all of that, how often (at what interval) you’ll want to take photos of the event.
Normally, you know the time length of your scene or event and how long you want to playback the time-lapsed clip. However, you do don't know the interval for capturing shots (frames).
The Magic Formula:
Interval = Event Length (in seconds) / FPS Rate * Clip Length (in seconds)
There are many applications for time-lapse photography, however, only few of them enable you setting the interval in easy way by 3 simple parameters:
(*) Event Length
(*) Clip Length
(*) FPS Rate (25 for PAL, 30 for NTSC)
This application will help you to bridge this gap and when you start your time-lapse project, I'd really recommend you to use this tool and plan ahead your shooting interval.