One of the niftiest ways I've seen this done was some software I used circa 2000 (I don't remember the name). It would create a variable-rate timelapse by saving a frame every time the image changed more than $x percent, calculated as the sum of differences of pixels from the previous frame, or thereabouts.

If someone was walking across the yard it would save every frame. The movement of the sun would move shadows enough to trigger a new image every few minutes. A bug flying past was small enough that it wouldn't trigger anything. The result was you could get a short video of everything interesting that happened through the day: shadows of trees sliding over the ground, every frame of the car pulling out of the driveway, shadows sliding over the ground some more, cat walks across the yard then lays down, shadows pan around more while the cat sits still, cat gets up and walks away, shadows pan around until the delivery guy comes...

It was an incredibly low-CPU way to see everything that happened without missing anything, and without having to fine-tune the motion detection very much. You just mask out any areas with constant motion, then adjust the slider for how much change triggered the next frame, which would let you adjust how fast the timelapse would go during the boring parts.

I've always wondered why the technique never became widespread.