It is recommended to take a break every couple of months from the regular use of herbal medicine. Many naturopaths will recommend a herbal medicine be taken for 8 weeks and then stopped. The therapist will then review and re-assess the individuals case and a new treatment plan may then be implemented. It is recommended to make a consultation with a Healthcare professional who can tailor make a regime for the individual, observe changes in health, treat according to the medical history, medications and family history. It is unfortunate that many people these days are self diagnosing and self-prescribing supplements. Taking responsibility for ones own health is something to be admired, however better, safer and more cost effective treatments can be established via a holistic practitioner.
In regards to your question concerning long term use of herbal medicines, the effect of a herbal medicine should be received after 2-3 months, however this is only if the diagnosis is correct in the first place as well as potency and dosage of the herb. Like all herbs, minerals, pharmaceuticals, supplements and nutrients toxic doses can be reached when they are taken to excessive doses, adverse reactions may then occur.