Dan Provost wrote on Studio Neat about the iOS 8 time-lapse feature.

This is an efficient way to assemble a time-lapse. When you start recording a time-lapse, the app only captures 2 frames per second. If the recording period extends beyond 10 minutes, the app switches to capturing only 1 frame per second, and deletes every other frame it had captured in the first 10 minutes. When the recording duration doubles (20 minutes), the same thing happens. Now the app is only capturing 1 frame every 2 seconds, and previous frames are dropped to match this tempo. And so forth. The longest video I recorded was 8 hours, but presumably using this method you could record for much longer (Apple’s website casually mentions 30 hours). Because the app is being so efficient with frame capture and storage, you don’t need to worry about your phone capacity filling up.

The result of this method is that anything you shoot will generally end up being between 20 and 40 seconds long, an ideal shareable length. Also worth mentioning, the resulting video is always 30 fps, the standard framerate for video. No surprises there.

It just works.