Other people have seen HAD lasting anywhere from a few hours to 20 hours at power levels ranging from 10 to 50 W. I do not recall examples of HAD below 1 W.
I take that back. Sort of. The text in Dardik says they saw 4 days of HAD at 0.6 W. The graph looks to me more like 3 W. See Fig. 6:
Another paper by this group shows HAD at 20 to 50 W, fluctuating, for about 20 hours.
As far as I know, the world record for HAD was Mizuno, 10 days starting at about 100 W:
I think this was the longest because the cathode was 100 g of Pd, which is much more than most cathodes.
There is no technological advantage to HAD. It is not desirable from an engineering point of view, any more than burning coal that you cannot extinguish.