I got mixed results.
I setup a 75-Gallon for cycling and about a week into the fishless cycle with only ammonia spikes, I added StressZyme. The next day I had a Nitrite spike, and within a day or so, I had a Nitrate spike. It rapidly cut my fishless cycle down. I've heard several people say theirs took 30+ days, mine took less than 2 weeks. (See the thread that contains my progress notes
here)
After my 75 had cycled and I added my fish to it, I tried cycling a 30-Gallon with StressZyme from the beginning, using a fishless cycle with no established decorations or filter media (the same conditions I had with my 75). I didn't get the same results, I got my ammonia spike, but instead of seeing it drop and have nitrites the next day, it took about a week for the ammonia to die down. Nitrites were cut down to 0 in about 2-3 days. The whole cycle process for the 30-Gallon seemed to take about 2 weeks.
All in all, I'd say the product I used, StressZyme, helped speed up the process, but I still had to wait quite a bit. If you're looking to Fishless Cycle, my best recommendation to you would be to get some established filter media from your LFS or a buddy in leiu of a cycling product.
