We generally recommend taking your herbal program for 100 days to give them a chance to make a great difference in you health.
That being said, the herbs will start to work as soon as you take your first dosage. In addition, it also depends the reason you are taking your herbs.
For instance, Bupleurum & Dragon Bone is a formula that will provide relief from stress on its first dosage.
But, if your Jing (Kidney/Adrenal) has become exhausted due to years of stress, it will take a few months to adequately replenish all that lost energy. That typically does not happen overnight.
There is one other phenomenon that people experience with herbs as well. When you initially take them, you may feel a real boost in energy but after a few months the herbs don't seem to give you the same boost.
However, when you go off the herbs, you may then notice a decrease in energy and realize you had just become used to your new baseline of greater energy.
It is similar to riding in a car. When the car first accelerates from 0-60, passengers feel the acceleration of being pushed back into the seats, but when the car is cruising along at 60 mph, there is no real feeling of going fast.
It can be the same with herbs. You will increase your levels of energy, and that will become the new normal feeling for you and you will forget what it was like before you took the herbs.