How to Calculate Frame Time for Your DSLR TimeLapse Movie
Your DSLR gives you some options for calculating time between frames. Either you can randomly select the duration between each frame, or you can determine it ahead of time, no pun intended, using a formula.
Say you’re trying to create a 30second timelapse movie of cars filling the parking lot before the big game. You’re already planning to capture the three hours it takes to fill up, but you’re just not sure about the duration. If you use the “guesstimate” rate of one frameper second, that would make your sequence around six minutes long.
Instead of guessing, try the following formula to let you know exactly how many frames you’ll need:

Determine the desired length of the sequence and use this formula:
(Desired duration in seconds) × (Frames per second for playback) = Amount of frames for playback
A 30second sequence that plays back at 30 frames per second would require 900 frames (30 × 30 = 900).

Next translate the total time into seconds with this formula:
(hours) × (60) × (60) = seconds
A threehour interval would use the following:
(3 hours) × (60 minutes) × (60 seconds) = 10,800 seconds.

Divide the time in seconds in Step 2 by the amount of required frames as calculated in Step 1 to come up with your frame duration.
(10,800) / (900) = 12 seconds.
To make a 30second movie, set the digital timer at one frame every 12 seconds and find something to do for the next three hours.