I take 12mg daily with TRT, it increases my free test on labs by 20-25% and works very well with zinc for increased libido. Not sure if it would be more effective or less for those not on exogenous test. That is my upper limit though because I've tried the old, "well if 12 is good, doubling it will be even better" and there was no further increase in free test, but an increase in e2.
As an added benefit, anecdotally, my brother races enduro and MX and the team doctors all up their boron intake along with D, K, and calcium when in training and race season because they found it does wonders for joints and tendon health and helps recovery when injured. My dad followed this protocol when he injured his shoulder in the gym and had surgery and the docs were amazed at how quickly he recovered at 75years old.