Above 5 ppm, or off the chart of the API test. Once nitrite starts to go down, the cycle is almost finished.
Personally I'd try to keep the nitrites on scale. I don't have hard numbers but I imagine that it is toxic to bacteria at some point.
If that takes a water change or two then that's OK, you don't lose any BB from doing a water change (the BB is all in your filter/gravel) and the hit to the cycle should be minimal if you continue to dose ammonia.
Nice thread Senor Discus .
I like your scientific approach towards the hobby .
Thanks Folio... Sorry I haven't posstes in a while with updates but I am out of town and should be back tomorrow to update the graph and all. I did a what I would call 80% WC on Saturday and then added 5 caps of ammonia to get it up to about 4 ppm as I wantes to stock up on the ammonia aka food for the Bb before I keft. I'm sure it will be around zero when I check tmrw. I will then add more ammonia and hopefully it will be home within 24 hours and I can add something to it haha. Will update soon!