Rotation is good, but for herbs at least it's more cost effective and more powerful to get a complex of multiple herbs, as herbs work synergestically together.
Planetary Formula's Bupleurum Liver Cleanse is the best one I've found. Dr. Cabot's is good too, but only at the outset for getting rid of environmental toxins, it's not a tonic. I also like Julia Chang's tinctures, but man they taste bitter!!!
You can take a bottle for as long as that takes, take a short break, buy another bottle and take that for as long as it takes, take a break. This is not an exact science, trust your intuition! or learn kinesiology or find a practioner that knows how to do applied kinesiology to be a bit more certain. :)