Hi Cherie,
This is a good question. 🙂
There are some guidelines that could help you decide how long you might need this blend (or specific herbs from this blend).
In certain situations there is a bit of a general guide: a person can benefit from taking herbal medicine/work on their health concern (with diet and lifestyle adjustments) for 1/2 the amount of time as the concern has already existed. So for example: if you have been experiencing fatigue for 1 year, then generally you could say that the tea would be of benefit for at least 6 months.
For the tea you are drinking, you could drink this anytime as a general nervous system support/nourishing tea for both prevention and for a already existing concern. If really long term, you might consider removing yarrow and adding something else gentle instead, like rose petals or lemon balm, or you could just leave it simple with nettle, oat and clover.
The other thought is that once you start to feel more nourished, energetic again, continue taking the tea for another couple of weeks just to make sure you are really nourished and back on track. 🙂
One more final thought is that you don’t have to stick to the same formula throughout. If this one is feeling too drying you can switch out one or two herbs, or simplify it anytime throughout, with the same goal in mind.
I hope this helps answer your question. If you have any further questions or clarification on this, do not hesitate to ask.
Warmly, Benna