Where the numbers come from
NoLight checks free space before it starts and every few seconds while it records. To do that it budgets 2,000,000 bytes a second at 1080p and 6,000,000 at 4K — about 7.2 GB and 21.6 GB an hour. Those budgets are set a little above what an iPhone usually writes, because the camera picks its own compression for each phone and scene, and it is better to warn early than to run out. So your real file will usually be smaller.
It won't start a recording with less than about a minute of room left, warns when five minutes are left, and stops cleanly with about twenty seconds to spare rather than writing the disk to zero.