So, having discovered gunpowder, my stone-age tribe needs some timed fuses for their explosive arrows and landmines. However, I am unsure of how accurate their fuses would be. The quality of their gunpowder isn't very high, and their rope is not very even in width or density, so it could take differing amounts of time to burn through. Approximately what will the variance in time for, for example, a fuse that is supposed to take 10 seconds to burn be? Additionally, what can the tribe do to create fuses that are more accurate?
Fuses don't typically rely on the qualities of rope; They burn a chemical soaked into the rope, often gunpowder.
So it is the consistency of the powder that matters. If you make a big batch and test some you should be able to guess fairly accurately, the stoneage concept of a second will probably have at least as much variation.