I didn't really know when to drop my nic... I just did it after a couple months, thinking it was appropriate. After dropping, I didn't notice much bodily difference - but I did use it more often. A few weeks later, I was craving an older higher nic flavor I had, and I was stunned to find that it nearly floored me after a few hits! ... okay, that's an exaggeration, but I got light headed and had the feeling of smoking too many cigarettes, if you know what I mean.
So for me, I dropped when the time felt right - and it's totally ambiguous. However, once I try the more potent juices, I can easily tell it's way too much! ... even if only a few milligrams!