These were on my 1998 BD, which has 80,360 miles on it.
Changed at 61k (~20K ago) preventative on Cyl 1/3 and corrective on Cyl 2/4 because 2 was leaking slightly. Because it was only 1K out of warranty, SOA gave me a $1K refund on my next Subaru purchase which came in handy when I bought the 2005 LGTW.
HGs have not been an issue on the 4th gens.
Pro dilectione Dei et patriam