Some things work immediately and don't need even a week or a day or an hour. It depends. With vitamin D3 I knew immediately; my energy and motivation was through the roof and the first night I couldn't sleep very much. With some that gave rise to a negative reaction, again, I knew immediately. With others, say B-complex, a week was sufficient to know that it increased my energy. It depends from person to person, from condition to condition, and so on and so forth.

But yes, if you mean long-term impact or healing of the nerves to be precise, that certainly takes a long time and is not at all obvious. So this is pretty much to echo what MrsD and Pinky have already said, including that over time the affect diminishes, say as with vitamin D in my case. It's all a fascinating, fun and art times frustrating experiment.

Good luck!