Time-lapse photography has one equation and it runs in both directions. Shots are the event length divided by the interval, and the clip is the shots divided by the frame rate.
shots = duration ÷ interval; clip seconds = shots ÷ frame rate
A two hour event at five second intervals is 1,440 frames, which at 24 frames a second plays for exactly one minute. That is a compression ratio of 120 to 1, and it will cost about 35.2 GB of card at 25 MB a shot. The time-lapse calculator reports all four figures at once, because they trade against each other and changing one changes the rest.
Work backwards from the clip
In practice you rarely know the interval you want. You know how long the finished clip should be, and the interval is the thing to solve for. A twenty second clip at 24 frames a second needs 480 frames, and 480 frames across two hours is one every fifteen seconds.
That is the calculation worth doing before the shoot rather than after it. Shooting every five seconds and discarding three quarters of the frames later works, but it costs three times the card space, three times the battery and three times the shutter actuations, and on a long interval shoot any of those can be the thing that ends the session early.
Long events need long intervals
Scale the event up and the arithmetic bites. An eight hour build at thirty second intervals gives 960 shots, 40 seconds of footage, and 23.4 GB. The compression ratio is 720 to 1, meaning each second of footage represents twelve minutes of real time.
That ratio is the honest measure of whether an interval suits the subject. Clouds at 120 to 1 look natural. A construction site at 720 to 1 looks natural too, because the subject moves slowly. Use 720 to 1 on something that moves at human speed and the result stutters, because consecutive frames no longer overlap in any way the eye can connect.
The shutter is the other half
Interval controls how often; shutter speed controls how much movement each frame contains. A frame exposed for a small fraction of its interval freezes motion and produces a staccato result, which is the usual complaint about time-lapse footage. The fix is a longer exposure per frame, which in daylight means cutting the light with a filter.
The ND filter calculator converts a base exposure into the one you get through a given filter strength, and shutter angle expresses the same relationship the way cinematographers do, as a fraction of the frame interval rather than as a time. The 180 degree convention, where the shutter is open for half the interval, exists for exactly this reason.
For the other end of the exposure scale, where the problem is holding the shutter open without the stars moving, there is the 500 rule.