hmm
well, that would be dependent on several things:
Size of the tank: A larger tank would be less affected by substrate then a smaller tank. Most substrates (unless well areated) only grow bacteria colonies on the top layer. In a small tank this would definitely be enough to cycle a tank, but in larger tanks, the bacteria in the substrate would have less and less of an effect on the time it takes to cycle. Whereas biomedia in proper filtration situations would cycle a larger tank much faster then substrate due to better access to food, oxygen, and larger impact on said tank.
Proper filtration: This would have a large impact on the speed of a cycle, most filters exagerate their performance, which is why many on this forum and others come close to doubling filtration if not more. If you have your aquarium properly filtered and move the biomedia over, it will cycle much faster then moving substrate over, again even more so in larger tanks.