Hey Piercy- I don't think there is a rule of thumb but it's a great question. The thing is, is that because if your live rock and the bacteria, you may never actually get a large ammonia spike. This is how I did it too, which is why I wanted to reply originally to your post. If you are seeing NITRATES, then your tank is converting ammonia. Nitrates also build slowly, which is why some cycles take weeks, even a month or more... Maybe check your water tests again today and post a pic of the results?