Wormwood is not one of the safer anti- parasite herbs, and is banned in many countries. Long term or high-dose use can lead to central nervous system damage. The book "Natural Guide to Medicinal Herbs and Plants," while acknowledging the hazards of long term Wormwood use, also mentions "the favorable actions of 3-4 week treatment," so that might be one good clue as to an appropriate time frame. I'd suggest using it until the bottle runs out or for a few weeks, whichever comes first. DO NOT use long term.