Ammonia is toxic to everything including the ammonia eating bacteria if the concentration is high enough. Keep it at anywhere between 2~6ppm should be fine for the fishless cycling. As the matter of fact, 6ppm was the number I was aiming for. I wasn't worried if it was slightly higher, but I'd be worried if it is off the scale. There is no need to worry about the ammonia concentration unless it is too high to even get an accurate reading from your test kit. No need to aim for any precise reading, just a general scale of between 2~6 will be perfect.
The reason I aimed for 6ppm is because I wanted to fully stock my aquarium as soon as the fishless cycling was done. I needed as much bacteria as possible in order to get rid of a lot of ammonia daily. If you don't plan to heavily stock your fish tank at once, there is no need to aim for 6ppm ammonia.
