With a simple interface Timelapse Helper allows you to calculate the shooting time needed and the playback duration of your timelapse movie.

Enter the interval in seconds that you'd like between photos the number of photos you'd like to limit the timelapse to or the frames-per-second of your timelapse film and the calculator will do the rest.

You can also set the calculator to interval-centric mode where the interval between photos changes in priority to the other fields. That way you can scroll the shooting time or playback time and see how that changes the interval between photos.