I will have published four articles and one book review in calendar year 2011. Our (all but non-existent) merit pay is based on the calendar year (strangely), so I think that this is an extraordinarily good year. Only 2009, when I published two monographs and one article, was better.
(And perhaps 1990 and 1994, when my two other books came out. In 1990 I also published in MLN.)
Even two articles a year is considered very good in my particular field. We don't tend to collaborate with dozens of other scholars at once or spin off endless short papers from the same experiment, so we don't produce paper after paper. If a tenure candidate had 12 articles in 6 years (plus an accepted book ms.), that would constitute a very strong case. Or if someone retired after a forty year career with 80 articles, that would be a pretty distinguished scholar. Two monographs (+ 8-12 articles posttenure) is the standard for promotion to full professor at a research university.
As I've argued before, it is actually harder to do a little bit of scholarship than to do a lot. If your aim is to produce and publish one article (on average a year), then it might be very difficult. The article might stall in the writing phase, or be rejected. It isn't likely to be as good, because it does not flow out of a more developed research program or a well-maintained scholarly base. Someone who doesn't have time for research during the academic year has difficulty switching gears in the summer and getting his fountain pens cleaned or removing the rust from her prose.